JOIN三个表 [英] JOIN three tables

问题描述

我试图检索两组信息（在一个查询中的图的红色和蓝色部分。

I am trying to retrieve two sets of information (the red and blue portions of the diagram in the one query.

我想我可以使用sql但如果TableC没有记录，它不返回任何内容，如果TableC没有记录，我仍然希望这个查询返回我的结果，如蓝色区域所示。

I thought I could do it using the sql as stated below but it does not return me anything when TableC has no records. If TableC has no records, I would still want this query to return me the results as indicated by the BLUE area.

SELECT A. * FROM TableA A

SELECT A.* FROM TableA A

JOIN TableB B ON（A.id = B.a_id）

JOIN TableB B ON (A.id = B.a_id)

JOIN TableC C ON（A.id = C.a_id）

JOIN TableC C ON (A.id = C.a_id)

感谢提前=]

推荐答案

尝试类似这样的

SELECT A.* FROM TableA A
LEFT OUTER JOIN TableB B ON (A.id = B.a_id)
LEFT OUTER JOIN TableC C ON (A.id = C.a_id)
WHERE B.a_id IS NOT NULL 
  OR c.a_id IS NOT NULL

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