MySQL数据库查询进阶之多表查询详解

MySQL数据库查询进阶之多表查询详解,第1张

MySQL数据库查询进阶之多表查询详解 目录
  • 一、多表查询
    • 1.引出
    • 2.笛卡尔积
    • 3. 笛卡尔积的解决方法
  • 二、多表查询分类
    • 1.等值连接和非等值连接
    • 2.自连接和非自连接
    • 3.内连接和外连接
    • 4.自然连接
    • 5.using连接
  • 三、子查询
    • 1.不相关子查询
    • 2.相关子查询
  • 四、聚合函数
    • 1.聚合函数介绍
      • 1.1 AVG和SUM函数
      • 1.2 MIN和MAX函数
      • 1.3 COUNT函数
    • 2.group by
      • 3.使用having进行分组后的筛选
      • 五、where和having的对比
        • 六、select的执行过程
          • 1.关键字顺序
            • 2.SELECT 语句的执行顺序
              • 3.SQL的执行原理(先了解)
              • 总结

                一、多表查询

                多表查询,也称为关联查询,指两个或更多个表一起完成查询 *** 作。

                前提条件:这些一起查询的表之间是有关系的(一对一、一对多),它们之间一定是有关联字段,这个关联字段可能建立了外键,也可能没有建立外键。比如:员工表和部门表,这两个表依靠“部门编号”进行关联。

                1.引出

                假如我们现在要查询员工的姓名还有部门名称

                这两个字段在不同表中,如果没有关联条件的话,查询出来的结果会怎么样呢,让我们来看看。

                SELECT last_name, department_name
                FROM employees, departments;
                +-----------+----------------------+
                | last_name | department_name      |
                +-----------+----------------------+
                | King      | Administration       |
                | King      | Marketing            |
                | King      | Purchasing           |
                | King      | Human Resources      |
                | King      | Shipping             |
                | King      | IT                   |
                | King      | Public Relations     |
                | King      | Sales                |
                | King      | Executive            |
                | King      | Finance              |
                | King      | Accounting           |
                | King      | Treasury             |
                ...
                | Gietz     | IT Support           |
                | Gietz     | NOC                  |
                | Gietz     | IT Helpdesk          |
                | Gietz     | Government Sales     |
                | Gietz     | Retail Sales         |
                | Gietz     | Recruiting           |
                | Gietz     | Payroll              |
                +-----------+----------------------+
                2889 rows in set (0.01 sec)
                
                
                SELECT COUNT(employee_id) FROM employees;
                #输出107行
                
                SELECT COUNT(department_id)FROM departments;
                #输出27行
                
                SELECT 107*27 FROM dual;
                107*27=2889
                

                很明显上面的 *** 作是错误的

                上面的 *** 作,会导致员工表的一条记录会和部门表的每一条记录相匹配,就好像一个员工在所有部门都工作过一样,从现实角度来说,很明显,是不会出现这种情况的,
                这种现象就是笛卡尔积。

                2.笛卡尔积

                笛卡儿积就是关系代数里的一个概念,表示两个表中的每一行数据任意组合的结果。比如:有两个表,左表有m条数据记录,x个字段,右表有n条数据记录,y个字段,则执行交叉连接后将返回m*n条数据记录,x+y个字段。笛卡儿积示意图如图所示。

                SQL92中,笛卡尔积也称为交叉连接,英文是 CROSS JOIN。在 SQL99 中也是使用 CROSS JOIN表示交叉连接。它的作用就是可以把任意表进行连接,即使这两张表不相关。在MySQL中如下情况会出现笛卡尔积:
                查询员工姓名和所在部门名称

                SELECT last_name,department_name FROM employees,departments;
                SELECT last_name,department_name FROM employees CROSS JOIN departments;
                SELECT last_name,department_name FROM employees INNER JOIN departments;
                SELECT last_name,department_name FROM employees JOIN departments;
                

                3. 笛卡尔积的解决方法

                笛卡尔积的错误会在下面条件下产生

                • 笛卡尔积的错误会在下面条件下产生

                  • 省略多个表的连接条件(或关联条件)
                  • 连接条件(或关联条件)无效
                  • 所有表中的所有行互相连接
                • 为了避免笛卡尔积, 可以在 WHERE 加入有效的连接条件。

                SELECT	table1.column, table2.column
                FROM	table1, table2
                WHERE	table1.column1 = table2.column2;  #连接条件
                
                #案例:查询员工的姓名及其部门名称
                SELECT last_name, department_name
                FROM employees, departments
                WHERE employees.department_id = departments.department_id;
                

                注意:如果不同的表中有相同的字段,我们要声明我们查的是哪一张表的字段,表名.字段名这个和Java中,类名.属性是类似的,挺好理解的。

                SELECT employees.last_name, departments.department_name,employees.department_id
                FROM employees, departments
                WHERE employees.department_id = departments.department_id;
                

                二、多表查询分类

                1.等值连接和非等值连接

                等值连接其实很好理解,就是谁等于谁的意思,使用=。
                非等值连接的话,比如查询某个字段>某个值的记录等等

                SELECT employees.employee_id, employees.last_name, 
                      employees.department_id, departments.department_id,
                    departments.location_id
                FROM   employees, departments
                WHERE  employees.department_id = departments.department_id;
                

                拓展:

                使用别名可以简化查询。— 有的字段名太长了列名前使用表名前缀可以提高查询效率。
                SELECT e.employee_id, e.last_name, e.department_id,
                d.department_id, d.location_id
                FROM   employees e , departments d
                WHERE  e.department_id = d.department_id;
                

                需要注意的是,如果我们使用了表的别名,在查询字段中、过滤条件中就只能使用别名进行代替,不能使用原有的表名,否则就会报错。

                2.自连接和非自连接

                自连接,它的字面意思就是自己和自己连接
                比如说现在有一张表,我们想要查找员工信息和对应的上级信息
                我们知道,只有一张表是没办法把它们关联起来的,要想把它们他们关联起来,肯定是要有关联条件的,那么就应该要有两张表,这个时候,我们就可以抽取出一张表,和本来的表本质上是一样的,然后我们对表起别名,table1和table2本质上是同一张表,只是用取别名的方式虚拟成两张表以代表不同的意义。然后两个表再进行内连接,外连接等查询。

                比如说:现在我们想要查找员工和对应老板的名字,我们就可以使用自连接

                SELECT CONCAT(worker.last_name ,' works for ' 
                   , manager.last_name)
                FROM   employees worker, employees manager
                WHERE  worker.manager_id = manager.employee_id ;
                

                练习:查询出last_name为 ‘Chen’ 的员工的 manager 的信息。

                3.内连接和外连接

                内连接: 合并具有同一列的两个以上的表的行, 结果集中不包含一个表与另一个表不匹配的行

                外连接: 两个表在连接过程中除了返回满足连接条件的行以外还返回左(或右)表中不满足条件的行 ,这种连接称为左(或右) 外连接。没有匹配的行时, 结果表中相应的列为空(NULL)。

                如果是左外连接,则连接条件中左边的表也称为主表,右边的表称为从表。

                如果是右外连接,则连接条件中右边的表也称为主表,左边的表称为从表。

                外连接查询的数据比较多

                SQL92:使用(+)创建连接

                在 SQL92 中采用(+)代表从表所在的位置。即左或右外连接中,(+) 表示哪个是从表。

                Oracle 对 SQL92 支持较好,而 MySQL 则不支持 SQL92 的外连接。

                #左外连接
                SELECT last_name,department_name
                 FROM employees ,departments 
                 WHERE  employees.department_id = departments.department_id(+);
                
                
                #右外连接 
                SELECT last_name,department_name 
                FROM employees ,departments 
                WHERE employees.department_id(+) = departments.department_id;  ```
                

                SQL99语法实现多表查询

                1.基本语法
                使用JOIN…ON子句创建连接的语法结构:

                SELECT table1.column, table2.column,table3.column FROM table1
                    JOIN table2 ON table1 和 table2 的连接条件
                        JOIN table3 ON table2 和 table3 的连接条件 
                

                语法说明:

                可以使用 ON 子句指定额外的连接条件 。

                这个连接条件是与其它条件分开的。ON 子句使语句具有更高的易读性。关键字 JOIN、INNER JOIN、CROSS JOIN 的含义是一样的,都表示内连接

                2.内连接(INNER JOIN)

                语法
                select 字段
                from 表1
                join 表2 on 两个表的连接条件
                where 其他子句

                比如我们现在想要查询各个部门的员工的信息,他们的连接条件就是员工表中部门id和部门表中的部门id一样

                SELECT e.employee_id, e.last_name, e.department_id, 
                       d.department_id, d.location_id
                FROM   employees e JOIN departments d
                ON     (e.department_id = d.department_id);
                这里截取部分结果
                +-------------+-------------+---------------+---------------+-------------+
                | employee_id | last_name   | department_id | department_id | location_id |
                +-------------+-------------+---------------+---------------+-------------+
                |         103 | Hunold      |            60 |            60 |        1400 |
                |         104 | Ernst       |            60 |            60 |        1400 |
                |         105 | Austin      |            60 |            60 |        1400 |
                |         106 | Pataballa   |            60 |            60 |        1400 |
                |         107 | Lorentz     |            60 |            60 |        1400 |
                |         120 | Weiss       |            50 |            50 |        1500 |
                |         121 | Fripp       |            50 |            50 |        1500 |
                |         122 | Kaufling    |            50 |            50 |        1500 |
                |         123 | Vollman     |            50 |            50 |        1500 |
                |         124 | Mourgos     |            50 |            50 |        1500 |
                |         125 | Nayer       |            50 |            50 |        1500 |
                |         126 | Mikkilineni |            50 |            50 |        1500 |
                |         127 | Landry      |            50 |            50 |        1500 |
                |         128 | Markle      |            50 |            50 |        1500 |
                |         129 | Bissot      |            50 |            50 |        1500 |
                

                使用内连接的一个问题就是他们把所有的信息都显示出来,它只能够显示匹配的数据,而外连接可以把不匹配的数据也显示出来

                先来看看表的数据,方便后续 *** 作

                mysql> select * from emp;
                +-------+--------+-----------+------+------------+---------+---------+--------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
                +-------+--------+-----------+------+------------+---------+---------+--------+
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
                +-------+--------+-----------+------+------------+---------+---------+--------+
                14 rows in set (0.00 sec)
                
                mysql> select * from dept;
                +--------+------------+----------+
                | DEPTNO | DNAME      | LOC      |
                +--------+------------+----------+
                |     10 | ACCOUNTING | NEW YORK |
                |     20 | RESEARCH   | DALLAS   |
                |     30 | SALES      | CHICAGO  |
                |     40 | OPERATIONS | BOSTON   |
                +--------+------------+----------+
                4 rows in set (0.00 sec)
                
                mysql> select * from emp e
                    -> join dept d
                    -> on e.deptno=e.deptno;
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | DNAME      | LOC      |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     10 | ACCOUNTING | NEW YORK |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     10 | ACCOUNTING | NEW YORK |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     10 | ACCOUNTING | NEW YORK |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     10 | ACCOUNTING | NEW YORK |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     10 | ACCOUNTING | NEW YORK |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     10 | ACCOUNTING | NEW YORK |
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     20 | RESEARCH   | DALLAS   |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     20 | RESEARCH   | DALLAS   |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     20 | RESEARCH   | DALLAS   |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     20 | RESEARCH   | DALLAS   |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     20 | RESEARCH   | DALLAS   |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     20 | RESEARCH   | DALLAS   |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     20 | RESEARCH   | DALLAS   |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     20 | RESEARCH   | DALLAS   |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     20 | RESEARCH   | DALLAS   |
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     30 | SALES      | CHICAGO  |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     30 | SALES      | CHICAGO  |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     30 | SALES      | CHICAGO  |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     40 | OPERATIONS | BOSTON   |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     40 | OPERATIONS | BOSTON   |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     40 | OPERATIONS | BOSTON   |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     40 | OPERATIONS | BOSTON   |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     40 | OPERATIONS | BOSTON   |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     40 | OPERATIONS | BOSTON   |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     40 | OPERATIONS | BOSTON   |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     40 | OPERATIONS | BOSTON   |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     40 | OPERATIONS | BOSTON   |
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                56 rows in set (0.01 sec)
                

                – 问题:
                – 1.40号部分没有员工,没有显示在查询结果中
                – 2.员工scott没有部门,没有显示在查询结果中
                所以想显示所有数据,要使用外连接

                外连接(OUTER JOIN)
                1.左外连接

                左外连接: left outer join – 左面的那个表的信息,即使不匹配也可以查看出效果
                SELECT 字段列表
                FROM A表 LEFT JOIN B表
                ON 关联条件
                WHERE 等其他子句;

                2.右外连接
                SELECT 字段列表
                FROM A表 RIGHT JOIN B表
                ON 关联条件
                WHERE 等其他子句;

                mysql> select *
                    -> from emp e
                    -> right outer join dept d
                    -> on e.deptno = d.deptno;
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | DNAME      | LOC      |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
                |  NULL | NULL   | NULL      | NULL | NULL       |    NULL |    NULL |   NULL |     40 | OPERATIONS | BOSTON   |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                15 rows in set (0.00 sec)
                

                3.满外连接(FULL OUTER JOIN)

                满外连接的结果 = 左右表匹配的数据 + 左表没有匹配到的数据 + 右表没有匹配到的数据。
                SQL99是支持满外连接的。使用FULL JOIN 或 FULL OUTER JOIN来实现。
                需要注意的是,MySQL不支持FULL JOIN,但是可以用 LEFT JOIN UNION RIGHT join代替。
                在讲满外连接之前,我们先来介绍一下union关键字的使用,相信看了以后大家就清楚了

                4.UNION

                合并查询结果

                利用UNION关键字,可以给出多条SELECT语句,并将它们的结果组合成单个结果集。合并时,两个表对应的列数和数据类型必须相同,并且相互对应。各个SELECT语句之间使用UNION或UNION ALL关键字分隔。

                语法格式:

                SELECT column,… FROM table1
                UNION [ALL]
                SELECT column,… FROM table2

                UNION *** 作符

                UNION *** 作符返回两个查询的结果集的并集,去除重复记录。

                `UNION ALL *** 作符

                UNION ALL *** 作符返回两个查询的结果集的并集。对于两个结果集的重复部分,不去重。

                注意:执行UNION ALL语句时所需要的资源比UNION语句少。如果明确知道合并数据后的结果数据不存在重复数据,或者不需要去除重复的数据,则尽量使用UNION ALL语句,以提高数据查询的效率。
                为什么union all的效率比较高呢?首先我们如果使用union的话,它会先把数据查询出来,紧接着还要进去去重 *** 作,它多了一步去重 *** 作,当然花费的时间就比较多了,影响效率。

                mysql> select *
                    -> from emp e
                    -> left outer join dept d
                    -> on e.deptno = d.deptno
                    -> union -- 并集 去重 效率低
                    -> select *
                    -> from emp e
                    -> right outer join dept d
                    -> on e.deptno = d.deptno;
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | DNAME      | LOC      |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  NULL | NULL   | NULL      | NULL | NULL       |    NULL |    NULL |   NULL |     40 | OPERATIONS | BOSTON   |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                15 rows in set (0.01 sec)
                
                mysql> ^C
                mysql> https://blog.csdn.net/weixin_42250835/article/details/123535439^Z^Z^C
                mysql> select *
                    -> from emp e
                    -> left outer join dept d
                    -> on e.deptno = d.deptno
                    -> union -- 并集 去重 效率低
                    -> select *
                    -> from emp e
                    -> right outer join dept d
                    -> on e.deptno = d.deptno;
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | DNAME      | LOC      |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  NULL | NULL   | NULL      | NULL | NULL       |    NULL |    NULL |   NULL |     40 | OPERATIONS | BOSTON   |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                15 rows in set (0.00 sec)
                
                mysql> select *
                    -> from emp e
                    -> left outer join dept d
                    -> on e.deptno = d.deptno
                    -> union all-- 并集 不去重 效率高
                    -> select *
                    -> from emp e
                    -> right outer join dept d
                    -> on e.deptno = d.deptno;
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | DNAME      | LOC      |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
                |  NULL | NULL   | NULL      | NULL | NULL       |    NULL |    NULL |   NULL |     40 | OPERATIONS | BOSTON   |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                29 rows in set (0.00 sec)
                

                为了让大家更清楚知道他们的区别,我们分别看一下有多少记录

                    -> on e.deptno = d.deptno' at line 2
                mysql> select *
                    -> from emp e
                    -> left outer join dept d
                    -> on e.deptno = d.deptno;
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | DNAME      | LOC      |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                14 rows in set (0.00 sec)
                
                mysql> select *
                    -> from emp e
                    -> right outer join dept d
                    -> on e.deptno = d.deptno;
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | DNAME      | LOC      |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
                |  NULL | NULL   | NULL      | NULL | NULL       |    NULL |    NULL |   NULL |     40 | OPERATIONS | BOSTON   |
                +-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
                15 rows in set (0.00 sec)
                

                14+15=29所=所以可以看出union all确实是不去重

                总结

                 中图:内连接 A∩B
                SELECT employee_id,last_name,department_name
                FROM employees e JOIN departments d
                ON e.`department_id` = d.`department_id`;
                
                 左上图:左外连接
                SELECT employee_id,last_name,department_name
                FROM employees e LEFT JOIN departments d
                ON e.`department_id` = d.`department_id`;
                
                 右上图:右外连接
                SELECT employee_id,last_name,department_name
                FROM employees e RIGHT JOIN departments d
                ON e.`department_id` = d.`department_id`;
                
                 左中图:A - A∩B
                SELECT employee_id,last_name,department_name
                FROM employees e LEFT JOIN departments d
                ON e.`department_id` = d.`department_id`
                WHERE d.`department_id` IS NULL
                
                 右中图:B-A∩B
                SELECT employee_id,last_name,department_name
                FROM employees e RIGHT JOIN departments d
                ON e.`department_id` = d.`department_id`
                WHERE e.`department_id` IS NULL
                
                 左下图:满外连接
                  左中图 + 右上图  A∪B
                SELECT employee_id,last_name,department_name
                FROM employees e LEFT JOIN departments d
                ON e.`department_id` = d.`department_id`
                WHERE d.`department_id` IS NULL
                UNION ALL  #没有去重 *** 作,效率高
                SELECT employee_id,last_name,department_name
                FROM employees e RIGHT JOIN departments d
                ON e.`department_id` = d.`department_id`;
                
                 右下图
                 左中图 + 右中图  A ∪B- A∩B 或者 (A -  A∩B) ∪ (B - A∩B)
                SELECT employee_id,last_name,department_name
                FROM employees e LEFT JOIN departments d
                ON e.`department_id` = d.`department_id`
                WHERE d.`department_id` IS NULL
                UNION ALL
                SELECT employee_id,last_name,department_name
                FROM employees e RIGHT JOIN departments d
                ON e.`department_id` = d.`department_id`
                WHERE e.`department_id` IS NULL
                

                4.自然连接

                SQL99 在 SQL92 的基础上提供了一些特殊语法,比如 NATURAL JOIN 用来表示自然连接。我们可以把自然连接理解为 SQL92 中的等值连接。它会帮你自动查询两张连接表中所有相同的字段,然后进行等值连接

                SELECT employee_id,last_name,department_name
                FROM employees e NATURAL JOIN departments d;
                

                上面的写法的效果和下面是一样的

                SELECT employee_id,last_name,department_name
                FROM employees e JOIN departments d
                USING (department_id);
                

                5.using连接

                当我们进行连接的时候,SQL99还支持使用 USING 指定数据表里的同名字段进行等值连接。但是只能配合JOIN一起使用。比如:

                SELECT employee_id,last_name,department_name
                FROM employees e JOIN departments d
                USING (department_id);
                

                你能看出与自然连接 NATURAL JOIN 不同的是,USING 指定了具体的相同的字段名称,你需要在 USING 的括号 () 中填入要指定的同名字段。同时使用 JOIN...USING 可以简化 JOIN ON 的等值连接。它与下面的 SQL 查询结果是相同的:

                SELECT employee_id,last_name,department_name
                FROM employees e ,departments d
                WHERE e.department_id = d.department_id;
                

                注意:using只能和join配合使用,而且要求两个关联字段在关联表中名称一致,而且只能表示关联字段值相等

                三、子查询

                1.不相关子查询

                子查询就是查询语句的嵌套,有多个select语句

                子查询的引入:

                – 查询所有比“CLARK”工资高的员工的信息

                – 步骤1:“CLARK”工资

                mysql> select * from emp where ename='clark';  工资2450
                +-------+-------+---------+------+------------+---------+------+--------+
                | EMPNO | ENAME | JOB     | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
                +-------+-------+---------+------+------------+---------+------+--------+
                |  7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL |     10 |
                +-------+-------+---------+------+------------+---------+------+--------+
                1 row in set (0.00 sec)
                

                – 步骤2:查询所有工资比2450高的员工的信息

                mysql> select * from emp where sal > 2450;
                +-------+-------+-----------+------+------------+---------+------+--------+
                | EMPNO | ENAME | JOB       | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
                +-------+-------+-----------+------+------------+---------+------+--------+
                |  7566 | JONES | MANAGER   | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
                |  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 | 2850.00 | NULL |     30 |
                |  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
                |  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
                |  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
                +-------+-------+-----------+------+------------+---------+------+--------+
                5 rows in set (0.01 sec)
                

                两次命令解决问题的话,效率低 ,第二个命令依托于第一个命令,第一个命令的结果给第二个命令使用,但是
                因为第一个命令的结果可能不确定要改,所以第二个命令也会导致修改
                将步骤1和步骤2合并 --》子查询:-- 一个命令解决问题 --》效率高

                mysql> select *from emp where sal>(select sal from emp where ename='clark');
                +-------+-------+-----------+------+------------+---------+------+--------+
                | EMPNO | ENAME | JOB       | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
                +-------+-------+-----------+------+------------+---------+------+--------+
                |  7566 | JONES | MANAGER   | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
                |  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 | 2850.00 | NULL |     30 |
                |  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
                |  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
                |  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
                +-------+-------+-----------+------+------------+---------+------+--------+
                5 rows in set (0.00 sec)
                

                【2】执行顺序:

                先执行子查询,再执行外查询;

                【3】不相关子查询:

                子查询可以独立运行,称为不相关子查询。

                【4】不相关子查询分类:

                根据子查询的结果行数,可以分为单行子查询和多行子查询。

                练习

                单行子查询

                mysql> -- 单行子查询
                mysql> -- 查询工资高与拼接工资的员工名字和工资
                mysql> select ename,sal from emp
                    -> where sal>(select avg(sal) from emp);
                +-------+---------+
                | ename | sal     |
                +-------+---------+
                | JONES | 2975.00 |
                | BLAKE | 2850.00 |
                | CLARK | 2450.00 |
                | SCOTT | 3000.00 |
                | KING  | 5000.00 |
                | FORD  | 3000.00 |
                +-------+---------+
                6 rows in set (0.00 sec)
                
                -- 查询和CLARK同一部门且比他工资低的雇员名字和工资。
                select ename,sal
                from emp
                where deptno = (select deptno from emp where ename = 'CLARK') 
                      and 
                      sal < (select sal from emp where ename = 'CLARK')
                +--------+---------+
                | ename  | sal     |
                +--------+---------+
                | MILLER | 1300.00 |
                +--------+---------+
                1 row in set (0.00 sec)
                
                 
                
                 多行子查询:
                  【1】查询【部门20中职务同部门10的雇员一样的】雇员信息。
                -- 查询雇员信息
                select * from emp;
                +-------+--------+-----------+------+------------+---------+---------+--------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
                +-------+--------+-----------+------+------------+---------+---------+--------+
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
                +-------+--------+-----------+------+------------+---------+---------+--------+
                14 rows in set (0.00 sec)
                -- 查询部门20中的雇员信息
                select * from emp where deptno = 20;
                +-------+-------+---------+------+------------+---------+------+--------+
                | EMPNO | ENAME | JOB     | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
                +-------+-------+---------+------+------------+---------+------+--------+
                |  7369 | SMITH | CLERK   | 7902 | 1980-12-17 |  800.00 | NULL |     20 |
                |  7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
                |  7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
                |  7876 | ADAMS | CLERK   | 7788 | 1987-05-23 | 1100.00 | NULL |     20 |
                |  7902 | FORD  | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
                +-------+-------+---------+------+------------+---------+------+--------+
                5 rows in set (0.00 sec)
                -- 部门10的雇员的职务:
                select job from emp where deptno = 10; -- MANAGER,PRESIDENT,CLERK
                +-----------+
                | job       |
                +-----------+
                | MANAGER   |
                | PRESIDENT |
                | CLERK     |
                +-----------+
                3 rows in set (0.00 sec)
                -- 查询部门20中职务同部门10的雇员一样的雇员信息。
                select * from emp 
                where deptno = 20 
                and job in (select job from emp where deptno = 10)
                -- > Subquery returns more than 1 row
                select * from emp 
                where deptno = 20 
                and job = any(select job from emp where deptno = 10)
                
                 【2】查询工资比所有的“SALESMAN”都高的雇员的编号、名字和工资。
                -- 查询雇员的编号、名字和工资
                select empno,ename,sal from emp
                +-------+--------+---------+
                | empno | ename  | sal     |
                +-------+--------+---------+
                |  7369 | SMITH  |  800.00 |
                |  7499 | ALLEN  | 1600.00 |
                |  7521 | WARD   | 1250.00 |
                |  7566 | JONES  | 2975.00 |
                |  7654 | MARTIN | 1250.00 |
                |  7698 | BLAKE  | 2850.00 |
                |  7782 | CLARK  | 2450.00 |
                |  7788 | SCOTT  | 3000.00 |
                |  7839 | KING   | 5000.00 |
                |  7844 | TURNER | 1500.00 |
                |  7876 | ADAMS  | 1100.00 |
                |  7900 | JAMES  |  950.00 |
                |  7902 | FORD   | 3000.00 |
                |  7934 | MILLER | 1300.00 |
                +-------+--------+---------+
                14 rows in set (0.00 sec)
                -- “SALESMAN”的工资:
                select sal from emp where job = 'SALESMAN';
                +---------+
                | sal     |
                +---------+
                | 1600.00 |
                | 1250.00 |
                | 1250.00 |
                | 1500.00 |
                +---------+
                4 rows in set (0.00 sec)
                -- 查询工资比所有的“SALESMAN”都高的雇员的编号、名字和工资。
                -- 多行子查询:
                select empno,ename,sal 
                from emp 
                where sal > all(select sal from emp where job = 'SALESMAN');
                +-------+-------+---------+
                | empno | ename | sal     |
                +-------+-------+---------+
                |  7566 | JONES | 2975.00 |
                |  7698 | BLAKE | 2850.00 |
                |  7782 | CLARK | 2450.00 |
                |  7788 | SCOTT | 3000.00 |
                |  7839 | KING  | 5000.00 |
                |  7902 | FORD  | 3000.00 |
                +-------+-------+---------+
                6 rows in set (0.00 sec)
                
                
                
                

                2.相关子查询

                【1】不相关的子查询引入:

                不相关的子查询:子查询可以独立运行,先运行子查询,再运行外查询。

                相关子查询:子查询不可以独立运行,并且先运行外查询,再运行子查询

                【2】不相关的子查询优缺点:

                好处:简单 功能强大(一些使用不相关子查询不能实现或者实现繁琐的子查询,可以使用相关子查询实现)

                缺点:稍难理解

                【3】sql展示:

                -- 【1】查询最高工资的员工  (不相关子查询)
                select * from emp where sal = (select max(sal) from emp)
                -- 【2】查询本部门最高工资的员工   (相关子查询)
                -- 方法1:通过不相关子查询实现:
                select * from emp where deptno = 10 and sal = (select max(sal) from emp where deptno = 10)
                union
                select * from emp where deptno = 20 and sal = (select max(sal) from emp where deptno = 20)
                union
                select * from emp where deptno = 30 and sal = (select max(sal) from emp where deptno = 30)
                -- 缺点:语句比较多,具体到底有多少个部分未知
                -- 方法2: 相关子查询
                select * from emp e where sal = (select max(sal) from emp where deptno = e.deptno) order by deptno
                -- 【3】查询工资高于其所在岗位的平均工资的那些员工  (相关子查询)
                -- 不相关子查询:
                select * from emp where job = 'CLERK' and sal >= (select avg(sal) from emp where job = 'CLERK')
                union ......
                -- 相关子查询:
                select * from emp e where sal >= (select avg(sal) from emp e2 where e2.job = e.job)

                四、聚合函数

                1.聚合函数介绍

                聚合函数作用于一组数据,并对一组数据返回一个值。

                聚合函数类型

                • AVG()
                • SUM()
                • MAX()
                • MIN()
                • COUNT()

                语法

                注意:聚合函数不允许嵌套使用

                1.1 AVG和SUM函数

                可以对数值型数据使用AVG 和 SUM 函数。

                他们在计算有空值的时候,会把非空计算进去,然后自动忽略空值
                AVG=SUM/COUNT

                mysql> select * from emp;
                +-------+--------+-----------+------+------------+---------+---------+--------+
                | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
                +-------+--------+-----------+------+------------+---------+---------+--------+
                |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
                |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
                |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
                |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
                |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
                |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |
                |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |
                |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |
                |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |
                |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
                |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
                |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
                |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
                |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
                +-------+--------+-----------+------+------------+---------+---------+--------+
                
                14 rows in set (0.00 sec)

                1.2 MIN和MAX函数

                可以对任意数据类型的数据使用 MIN 和 MAX 函数。

                1.3 COUNT函数

                COUNT(*)返回表中记录总数,适用于任意数据类型

                mysql> select count(*) from emp;
                +----------+
                | count(*) |
                +----------+
                |       14 |
                +----------+
                1 row in set (0.01 sec)
                

                计算指定字段再查询结果中出现的个数

                mysql> select count(comm) from emp;
                +-------------+
                | count(comm) |
                +-------------+
                |           4 |
                +-------------+
                1 row in set (0.00 sec)
                

                COUNT(expr) 返回expr不为空的记录总数。

                -问题:用count(*),count(1),count(列名)谁好呢?

                其实,对于MyISAM引擎的表是没有区别的。这种引擎内部有一计数器在维护着行数。

                Innodb引擎的表用count(*),count(1)直接读行数,复杂度是O(n),因为innodb真的要去数一遍。但好于具体的count(列名)。

                问题:能不能使用count(列名)替换count(*)?

                不要使用 count(列名)来替代 count(*)count(*)是 SQL92 定义的标准统计行数的语法,跟数据库无关,跟 NULL 和非 NULL 无关。

                说明: count(*)会统计值为 NULL 的行,而 count(列名)不会统计此列为 NULL 值的行。
                这样子讲的话,大家可能还比较懵,接下来,我来演示一下

                2.group by

                使用group by可以进行分组,我们以前使用avg可以求出所有员工的平均工资,但是如果我们想要求各个部门的员工的平均工资的话,就得对部门进行分组,以部门为单位来划分,然后求出他们各自的平均工资
                注意:字段不可以和多行函数一起使用,因为记录个数不匹配,这样就会导致查询的数据没有全部展示,但是,如果这个字段属于分组是可以的

                mysql> select deptno,avg(sal) from emp group by deptno;
                +--------+-------------+
                | deptno | avg(sal)    |
                +--------+-------------+
                |     20 | 2175.000000 |
                |     30 | 1566.666667 |
                |     10 | 2916.666667 |
                +--------+-------------+
                3 rows in set (0.00 sec)
                
                统计各个岗位的平均工资
                mysql> select job,avg(sal) from emp group by job;
                +-----------+-------------+
                | job       | avg(sal)    |
                +-----------+-------------+
                | CLERK     | 1037.500000 |
                | SALESMAN  | 1400.000000 |
                | MANAGER   | 2758.333333 |
                | ANALYST   | 3000.000000 |
                | PRESIDENT | 5000.000000 |
                +-----------+-------------+
                5 rows in set (0.00 sec)
                

                3.使用having进行分组后的筛选

                使用having的条件:

                1 行已经被分组。

                2. 使用了聚合函数。

                3. 满足HAVING 子句中条件的分组将被显示。

                4. HAVING 不能单独使用,必须要跟 GROUP BY 一起使用。

                统计各个部门的平均工资 ,只显示平均工资2000以上的 - 分组以后进行二次筛选 having

                mysql> select deptno,avg(sal) from emp
                    -> group by deptno
                    -> having avg(sal) >2000;
                +--------+-------------+
                | deptno | avg(sal)    |
                +--------+-------------+
                |     20 | 2175.000000 |
                |     10 | 2916.666667 |
                +--------+-------------+
                2 rows in set (0.01 sec)
                
                

                五、where和having的对比

                区别1:WHERE 可以直接使用表中的字段作为筛选条件,但不能使用分组中的计算函数作为筛选条件;HAVING 必须要与 GROUP BY 配合使用,可以把分组计算的函数和分组字段作为筛选条件。

                这决定了,在需要对数据进行分组统计的时候,HAVING 可以完成 WHERE 不能完成的任务。这是因为,在查询语法结构中,WHERE 在 GROUP BY 之前,所以无法对分组结果进行筛选。HAVING 在 GROUP BY 之后,可以使用分组字段和分组中的计算函数,对分组的结果集进行筛选,这个功能是 WHERE 无法完成的。另外,WHERE排除的记录不再包括在分组中。

                区别2:如果需要通过连接从关联表中获取需要的数据,WHERE 是先筛选后连接,而 HAVING 是先连接后筛选。 这一点,就决定了在关联查询中,WHERE 比 HAVING 更高效。因为 WHERE 可以先筛选,用一个筛选后的较小数据集和关联表进行连接,这样占用的资源比较少,执行效率也比较高。HAVING 则需要先把结果集准备好,也就是用未被筛选的数据集进行关联,然后对这个大的数据集进行筛选,这样占用的资源就比较多,执行效率也较低。

                小结如下:

                开发中的选择:

                WHERE 和 HAVING 也不是互相排斥的,我们可以在一个查询里面同时使用 WHERE 和 HAVING。包含分组统计函数的条件用 HAVING,普通条件用 WHERE。这样,我们就既利用了 WHERE 条件的高效快速,又发挥了 HAVING 可以使用包含分组统计函数的查询条件的优点。当数据量特别大的时候,运行效率会有很大的差别。

                六、select的执行过程

                1.关键字顺序

                SELECT … FROM … WHERE … GROUP BY … HAVING … ORDER BY … LIMIT…

                2.SELECT 语句的执行顺序

                FROM -> WHERE -> GROUP BY -> HAVING -> SELECT 的字段 -> DISTINCT -> ORDER BY -> LIMIT

                比如你写了一个 SQL 语句,那么它的关键字顺序和执行顺序是下面这样的:

                SELECT DISTINCT player_id, player_name, count(*) as num  顺序 5
                FROM player JOIN team ON player.team_id = team.team_id   顺序 1
                WHERE height > 1.80  顺序 2
                GROUP BY player.team_id   顺序 3
                HAVING num > 2  顺序 4
                ORDER BY num DESC   顺序 6
                LIMIT 2   顺序 7
                

                3.SQL的执行原理(先了解)

                SELECT 是先执行 FROM 这一步的。在这个阶段,如果是多张表联查,还会经历下面的几个步骤:

                1. 首先先通过 CROSS JOIN 求笛卡尔积,相当于得到虚拟表 vt(virtual table)1-1;
                2. 通过 ON 进行筛选,在虚拟表 vt1-1 的基础上进行筛选,得到虚拟表 vt1-2;
                3. 添加外部行。如果我们使用的是左连接、右链接或者全连接,就会涉及到外部行,也就是在虚拟表 vt1-2 的基础上增加外部行,得到虚拟表 vt1-3。

                当然如果我们 *** 作的是两张以上的表,还会重复上面的步骤,直到所有表都被处理完为止。这个过程得到是我们的原始数据。

                当我们拿到了查询数据表的原始数据,也就是最终的虚拟表 vt1,就可以在此基础上再进行 WHERE 阶段。在这个阶段中,会根据 vt1 表的结果进行筛选过滤,得到虚拟表 vt2。

                然后进入第三步和第四步,也就是 GROUP 和 HAVING 阶段。在这个阶段中,实际上是在虚拟表 vt2 的基础上进行分组和分组过滤,得到中间的虚拟表 vt3 和 vt4。

                当我们完成了条件筛选部分之后,就可以筛选表中提取的字段,也就是进入到 SELECT 和 DISTINCT 阶段。

                首先在 SELECT 阶段会提取想要的字段,然后在 DISTINCT 阶段过滤掉重复的行,分别得到中间的虚拟表 vt5-1 和 vt5-2。

                当我们提取了想要的字段数据之后,就可以按照指定的字段进行排序,也就是 ORDER BY 阶段,得到虚拟表 vt6。

                最后在 vt6 的基础上,取出指定行的记录,也就是 LIMIT 阶段,得到最终的结果,对应的是虚拟表 vt7。

                当然我们在写 SELECT 语句的时候,不一定存在所有的关键字,相应的阶段就会省略。

                同时因为 SQL 是一门类似英语的结构化查询语言,所以我们在写 SELECT 语句的时候,还要注意相应的关键字顺序,所谓底层运行的原理,就是我们刚才讲到的执行顺序。

                总结

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