mysql 子查询 null或者分组无效导致的查询结果空

直入正题

建表

CREATE TABLE user (
id BIGINT(20) NOT NULL AUTO_INCREMENT,
userName VARCHAR(50) NULL DEFAULT NULL,
userPassword VARCHAR(50) NULL DEFAULT NULL,
PRIMARY KEY (id)
)
COLLATE=‘utf8_general_ci’
ENGINE=InnoDB
AUTO_INCREMENT=4
;

造数据

INSERT INTO user (id, userName, userPassword) VALUES (1, ‘张三’, ‘123’);
INSERT INTO user (id, userName, userPassword) VALUES (2, ‘张三’, ‘3’);
INSERT INTO user (id, userName, userPassword) VALUES (3, ‘李四’, NULL);
INSERT INTO user (id, userName, userPassword) VALUES (4, ‘李四’, NULL);
INSERT INTO user (id, userName, userPassword) VALUES (5, ‘aa’, ‘123’);
INSERT INTO user (id, userName, userPassword) VALUES (6, ‘aa’, NULL);
INSERT INTO user (id, userName, userPassword) VALUES (7, ‘bb’, NULL);

如下sql由于not in子查询结果有null，导致结果行空

## not in() 子查询结果不能有null，解决办法 加条件 t.userPassword is not null
select * from user t

where t.userPassword not in 
(
select t.userPassword from user t where t.userName='aa'
)

子查询gropu by 导致结果集空。

如下：

select * from user t

where t.id not in 
(
select t.id from user t 
group by t.userName
)

如图select t.id from user t group by t.userName 子查询结果是4条数据，按理最终结果行是三条。

但是结果行是空，如下：

原因：子查询直接用gropu by,sql解析器直接获取结果集，并不会gropu by，所以子查询得到所有结果。主sql not in (所有结果），自然得到空。

通过再嵌套解决该问题，修改如下：

select * from user t

where t.id not in 
(
	select id
	from (
		select t.id from user t 
		group by t.userName
	)tt

)

得到正确结果：