MySQL 别名子查询

Question

我有一个只包含两列的表：id1 和 id2。 MySQL 中的以下查询可以正常工作：

(select id1 as id, count(id1) as cnt
from my_table 
group by id1)
union all
(select id2 as id, count(id2) as cnt
from my_table
group by id2)

如果我想将上面的查询用作子查询，我需要为每个派生表设置别名 - 下面的代码会出错（“每个派生表都必须有自己的别名”）：

select id, cnt from
(select id1 as id, count(id1) as cnt
from my_table 
group by id1)
union all
(select id2 as id, count(id2) as cnt
from my_table
group by id2)

但是我找不到正确的语法来给派生表起别名。我正在尝试添加和删除括号，但到目前为止还没有运气。对于下面的查询，我只是得到一般的 SQL 语法错误：

select id, cnt from
(select id1 as id, count(id1) as cnt
from my_table 
group by id1) as tab1
union all
(select id2 as id, count(id2) as cnt
from my_table
group by id2) as tab2

稍后我会想用这个子查询做更多的事情，而不是只选择 id 和 cnt，但它是一个简化的场景，可以找到别名子查询的正确语法。

Answer 1

试试下面：因为你合并了所有的结果，所以你只需要一个别名

SELECT id, cnt
FROM
(
    SELECT id1 AS id, COUNT(id1) AS cnt
    FROM my_table 
    GROUP BY id1
    UNION ALL
    SELECT id2, COUNT(id2)
    FROM my_table
    GROUP BY id2
) AS tab;

Answer 2

为了完善这个问题，以下是合并两个派生表所需的语法：

SELECT id, cnt
FROM
(
    SELECT id1 AS id, COUNT(id1) AS cnt
    FROM my_table 
    GROUP BY id1
) AS tab1
UNION ALL
SELECT id, cnt
FROM
(
    SELECT id2 AS id, COUNT(id2) AS cnt
    FROM my_table
    GROUP BY id2
) AS tab2;

Demo

您需要为UNION 中子查询的每个 组件显式选择语句。当然，@fa06 的答案是你应该使用的，但是如果你需要沿着这条路走，上面就是你如何让它工作的方法。