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mysql delete语句删除数据

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Summary: in this tutorial, you will learn how to use the MySQL DELETE statement to remove data from one ore more database tables.

To remove data from a table, you use the MySQL DELETEstatement. The MySQL DELETE statement allows you to remove records from not only one table but also multiple tables using a single DELETE statement.

MySQL delete from one table

To remove data from a single table, you use the following DELETE statement:

DELETE FROM table   
[WHERE conditions] [ORDER BY ...] [LIMIT rows]

Followed the DELETE FROM clause is the table name that you want to delete records.

The WHERE clause specifies which rows you want to delete. If a record meets the WHERE condition, it is deleted permanently from the table. If you omit the WHERE clause, all records in the table are deleted.

The DELETE statement returns the number of rows deleted specified by the ROW_COUNT() function.

Notice that the ROW_COUNT() function returns the number of rows inserted, updated or deleted by the last INSERT, UPDATE or DELETE statement

MySQL delete from a table examples

Suppose you want to remove employees whose officeNumberis 4, you use the DELETE statement with the WHERE clause as the following query:

DELETE FROM employees 
WHERE officeCode = 4

To delete all employee records from the employees table, you use the DELETE statement without the WHERE clause as follows:

DELETE FROM employees

All employee records in the employees table were deleted.

MySQL delete from multiple tables

To delete records from multiple tables, you use one of the following DELETE statements:

DELETE table_1, table_2,...
FROM table-refs   
[WHERE conditions]

DELETE FROM table_1, table_2,...
USING table-refs
[WHERE conditions]

Let’s examine each statement in greater detail:

  • Both DELETE statements delete records from multiple tables i.e., table_1, table_2,… specified after the DELETE keyword.
  • The first DELETE statement uses the FROM clause while the second one use the USING clause.
  • The WHERE clause is used to determine which record to be deleted.
  • Both statements return the number of rows deleted from the multiple tables.

MySQL delete from multiple tables example

Let’s take a look at the offices and employees tables in the MySQL sample database.

mysql delete multiple tables offices employees

Suppose one office is closed and you want to remove all employee records in the employees table associated with that office and also the office itself in the offices table, you can use the second form of the MySQL DELETE statement

The following query removes the office record with the code is 1 in the offices table and also all employee records associated with the office 1 in the employees table:

DELETE employees, 
	   offices 
FROM employees, 
	 offices 
WHERE employees.officeCode = offices.officeCode  AND 
      offices.officeCode = 1

You can verify the changes by using the following SELECT statements to query data from both employees table and offices table.

SELECT * FROM employees 
WHERE officeCode = 1;

SELECT * FROM offices
WHERE officeCode = 1;

You can achieve the same effect by using the second form of the MySQL DELETE statement as follows:

DELETE FROM employees, offices 
USING employees, offices 
WHERE employees.officeCode = offices.officeCode  AND 
      offices.officeCode = 1

In this tutorial, you have learned various forms of the MySQL DELETE statement to delete records from one or more tables.

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