为什么带括号/括号的UNION SELECT返回不同的总行数

Question

当我运行此查询时：

SELECT 1, `name` FROM `table1` WHERE `reg` = 3 AND `lang` = 5
UNION
SELECT 2, `name` FROM `table1` WHERE `reg` = 1 AND `lang` = 2
UNION
SELECT 3, `name` FROM `table1` WHERE `reg` = 6 AND `lang` = 10

返回：Showing rows 0 - 24 (435 total, Query took 0.0110 sec)

我需要使用LIMIT 5运行这些查询，因此我必须在每个select周围使用括号，在这种情况下使用括号和限制：

(SELECT 1, `name` FROM `table1` WHERE `reg` = 3 AND `lang` = 5 LIMIT 5)
UNION
(SELECT 2, `name` FROM `table1` WHERE `reg` = 1 AND `lang` = 2 LIMIT 5)
UNION
(SELECT 3, `name` FROM `table1` WHERE `reg` = 6 AND `lang` = 10 LIMIT 5)

返回以下内容：Showing rows 0 - 24 (17098 total, 0 in query, Query took 0.0020 sec)

我做错了吗？

Answer 1

您的第二个查询与第一个查询不同，因为您在第二个查询中使用 LIMIT 5

修改： -

你可以试试这个：

(SELECT 1 as sort_col, `name` FROM `table1` WHERE `reg` = 3 AND `lang` = 5  LIMIT 5)
UNION
(SELECT 2, `name` FROM `table1` WHERE `reg` = 1 AND `lang` = 2 LIMIT 5)
UNION
(SELECT 3, `name` FROM `table1` WHERE `reg` = 6 AND `lang` = 10  LIMIT 5)
ORDER BY sort_col, `name`

来自MYSQL Docs：

  

使用ORDER BY或LIMIT子句对整个UNION进行排序或限制   结果，括起单个SELECT语句并放置   最后一个之后的ORDER BY或LIMIT。

Answer 2

使用括号更改查询的处理，为每个子查询分配优先级

调查两个查询的使用Explain YOUR_QUERY。