将2个SQL SELECT结果集合并为一个 [英] Joining 2 SQL SELECT result sets into one

问题描述

我有2条select语句，返回的数据如下:

I've got 2 select statements, returning data like this:

Select 1
col_a   col_b

Select 2
col_a   col_c

如果我参加工会，我会得到类似的东西

If I do union, I get something like

col_a col_b

行合并.我需要的是这样的:

And rows joined. What i need is getting it like this:

col_a  col_b  col_c

已加入col_a

推荐答案

使用JOIN连接子查询，并使用ON表示每个子查询中的行必须匹配的位置:

Use JOIN to join the subqueries and use ON to say where the rows from each subquery must match:

SELECT T1.col_a, T1.col_b, T2.col_c
FROM (SELECT col_a, col_b, ...etc...) AS T1
JOIN (SELECT col_a, col_c, ...etc...) AS T2
ON T1.col_a = T2.col_a

如果T1中有一些col_a值，而T2中没有，则可以改用LEFT OUTER JOIN.

If there are some values of col_a that are in T1 but not in T2, you can use a LEFT OUTER JOIN instead.

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