MySQL中条件放在where后面与放在on后面的区别

Posted 2021-01-01 Acode

假设有两种表：test_on_position表和address表，address表存放地址，test_on_position存放会员及其常用的地址，数据如下：

address表：

test_on_position表：

1. left join

条件在where后面

SELECT
	*
FROM
	test_on_position
LEFT JOIN address ON test_on_position.address = address.id
WHERE
	test_on_position.address IS NOT NULL

运行结果：

条件在on后面

SELECT
	*
FROM
	test_on_position
LEFT JOIN address ON test_on_position.address = address.id AND test_on_position.address IS NOT NULL

运行结果：

结果分析

当条件放在on后面时，无论on条件的真假，都会返回左表的值；

当条件放在where后面时，只有满足条件的记录会返回。

inner join

条件在on后面

SELECT
	*
FROM
	test_on_position
INNER JOIN address ON test_on_position.address = address.id AND test_on_position.address IS NOT NULL

运行结果：

条件在where后面

SELECT
	*
FROM
	test_on_position
INNER JOIN address ON test_on_position.address = address.id
WHERE
	test_on_position.address IS NOT NULL

运行结果：

结果分析

无论条件放在哪里，都只会返回满足条件的结果。