MySQL 别名子查询

Question

我有一个 table 仅包含两列：id1 和 id2。 MySQL 中的以下查询工作正常：

(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，我需要为每个派生的 table 添加别名 - 下面的代码给出错误 ("every derived table must have it’s own alias"):

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)

但我找不到正确的语法来为派生的 table 设置别名。我正在尝试添加和删除括号，但到目前为止还没有成功。对于下面的查询，我只得到通用的 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

稍后我想用这个子查询做更多的事情，而不是只选择 id 和 cnt，但这是为别名子查询找到正确语法的简化方案。

Answer 1

试试下面的方法：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

为了解决这个问题，这里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

对于 UNION 中子查询的每个组件，您需要明确的 select 语句。当然，@fa06 的回答是你应该使用的，但如果你需要走这条路，上面是你如何让它工作。

MySQL 别名子查询

MySQL aliasing subqueries

mysql

sql

aliases

Demo