如何用2个以上的桌子进行LEFT JOIN?

问题描述

当前我正在执行以下查询:

Currently I am doing this query:

select a.x, b.x, c.x
from number as a, customer as b, numbergroup as c
where a.b = b.b and a.c = c.c and c.b = b.b

但是，我想从表"a"中检索记录，即使"a.c = null"，由于"a"和"c"之间的连接而无法检索.

However, I want to retrieve records from table "a" even if "a.c = null", which is not retrieved due to the join between "a" and "c".

我找到了有关 left join 的信息，但我不知道如何当查询涉及两个以上的表时(例如在这种情况下)，可以执行此操作.

I have found information about left join but I don't know how to do it when the query involves more than two tables like in this case.

推荐答案

select a.x, b.x, c.x 
from number as a
left join customer as b on a.b = b.b
left join numbergroup as c on a.c = c.c and c.b = b.b

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