mysql中的union和order by、limit
我有一个表
CREATE TABLE `test1` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(20) NOT NULL,
`desc` varchar(100) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
(1)以下查询会报错误:[Err] 1221 - Incorrect usage of UNION and ORDER BY
select * from test1 where name like 'A%' order by name
union
select * from test1 where name like 'B%' order by name
应改为:
select * from test1 where name like 'A%'
union
select * from test1 where name like 'B%' order by name
因为union中,在不用括号的情况下,只能用一个order by(想一想,如果union两边的order by的列名不一样会怎么样),这会对union后的结果集进行排序
或者改为:
(select * from test1 where name like 'A%' order by name)
union
(select * from test1 where name like 'B%' order by name)
这两个order by在union前进行
(2)同样的
select * from test1 where name like 'A%' limit 10
union
select * from test1 where name like 'B%' limit 20
相当于
(select * from test1 where name like 'A%' limit 10)
union
(select * from test1 where name like 'B%') limit 20
即后一个limit作用于的是union后的结果集,而不是union后的select
也可以加括号来得到你想要的结果
(select * from test1 where name like 'A%' limit 10)
union
(select * from test1 where name like 'B%' limit 20)
(3)UNION和UNION ALL区别
union会过滤掉union两边的select得到的结果集中的重复的行,而union all不会过滤掉重复的行