ORDER BY和LIMIT之后的UNION

Question

我的目标是执行两个不同的查询，然后将它们组合起来 我的代码是：

SELECT * FROM some tables WHERE ... ORDER BY field1 LIMIT 0,1 
UNION   
SELECT * FROM some tables WHERE ...

我收到以下错误：

  

＃1221 - UNION和ORDER BY的使用不正确

ORDER BY仅适用于第一个查询。我该如何执行此任务？

Answer 1

您可以使用括号允许在单个查询中使用ORDER / LIMIT：

(SELECT * FROM some tables WHERE ... ORDER BY field1 LIMIT 0, 1)
UNION   
(SELECT * FROM some tables WHERE ...)
ORDER BY 1   /* optional -- applies to the UNIONed result */
LIMIT 0, 100 /* optional -- applies to the UNIONed result */

Answer 2

SELECT * FROM (SELECT * FROM some tables WHERE ... ORDER BY field1 LIMIT 0,1) x
UNION ALL
SELECT * FROM some tables WHERE ...

请注意使用UNION ALL：

• UNION在执行此操作之前从结果集中删除重复的行，并且数据库命令所有行（因此整个结果集已排序）
• UNION ALL保留订单和重复项

Answer 3

将所有内容放在圆括号中：

(SELECT * FROM table1 ORDER BY datetime  )
UNION   
(SELECT * FROM table2 ORDER BY datetime DESC)

Answer 4

(SELECT user_id AS id FROM tbl_user)
UNION
(SELECT address_id AS id FROM tbl_address)
ORDER BY id ASC LIMIT 10