链接分为内连接,外连接,交叉连接等,外连接分为左外,右外连接
案例:
内链接
select * from student a
inner join stumarks b
on a.stuid=b.stuid
外连接
select * from student a
left join stumarks b
on a.stuid=b.stuid
1、创建测试表;
create table company(companyId number, companyName varchar2(20), contacts varchar2(20))
create table users(userId number, companyId number, userName varchar2(20), userAge number, address varchar2(20))
2、插入测试数据;
insert into company values (1,'腾讯科技','马化腾')
insert into company values (2,'微软公司','鲍威尔')
insert into company values (3,'新浪科技','小明' )
insert into users values (1,1,'jack',23 , '上海')
insert into users values (2,1,'jack2', 23, '上海')
insert into users values (3,2,'jack3', 23, '上海')
insert into users values (4,2,'jack4', 23, '上海')
insert into users values (5,3,'jack5', 23, '上海')
insert into users values (6,3,'jack6', 23, '上海')
3、查询表中数据;
select * from users
4、编写sql,关联两张表;select * from users t , company b where t.companyid = b.companyid and t.companyid = 1
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