MySQL Select 组合唯一

Question

Table: Contacts
id    | name  | has_this
------------------------
1     | Jeff  | 0
2     | Terry | 1
3     | Tom   | 0
4     | Henry | 1

Table: has_thing
id    | owner | thing
---------------------
1     | Terry | stuff
2     | Tom   | stuff
3     | Toby  | stuff

我想要一个 SELECT return

name  | thing
-------------
Terry | stuff
Tom   | stuff
Henry | 
Toby  | stuff

基本上，我想我想要一个 JOIN，但我希望 table 2(has_thing) 中不在 table 1 中的任何名称都包含在输出中，并且table 1(Contacts) WHERE has_this=1 中的任何名称都将包含在输出

中

Answer 1

SELECT name, MAX(thing) as thing
FROM (SELECT c.name, h.thing
      FROM Contacts AS c
      JOIN has_thing AS h ON c.name = h.name
      UNION
      SELECT name, ''
      FROM Contacts
      WHERE has_thing = 1) AS subquery
GROUP BY name

MAX(thing) 确保我们在联系人具有 has_thing = 1.

时从第一个查询中获取非空 thing

你也可以用 LEFT JOIN:

SELECT c.name, IFNULL(h.thing, '') AS thing
FROM Contacts AS c
LEFT JOIN has_thing AS h ON c.name = h.name
WHERE c.has_thing = 1
OR h.name IS NOT NULL

MySQL Select 组合唯一

MySQL Select Combined Unique

mysql

select

join