如何在 SQL 中使用多个 LEFT JOIN？
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How to use multiple LEFT JOINs in SQL?
提问by cute
Is it possible to use multiple left joins in sql query?
是否可以在 sql 查询中使用多个左连接？
LEFT JOIN
ab
ON
ab.sht = cd.sht
i want to add to attach one more query like this to it? will it work?
我想添加一个这样的查询吗？它会起作用吗？
LEFT JOIN
ab AND aa
ON
ab.sht = cd.sht
AND
aa.sht = cc.sht
Will this work?
这会起作用吗？
回答by btilly
Yes it is possible. You need one ON for each join table.
对的，这是可能的。每个连接表都需要一个 ON。
LEFT JOIN ab
ON ab.sht = cd.sht
LEFT JOIN aa
ON aa.sht = cd.sht
Incidentally my personal formatting preference for complex SQL is described in http://bentilly.blogspot.com/2011/02/sql-formatting-style.html. If you're going to be writing a lot of this, it likely will help.
顺便提一下，我个人对复杂 SQL 的格式偏好在http://bentilly.blogspot.com/2011/02/sql-formatting-style.html 中有所描述。如果你要写很多这样的东西，它可能会有所帮助。
回答by Daniel DiPaolo
Yes, but the syntax is different than what you have
是的，但语法与您所拥有的不同
SELECT
<fields>
FROM
<table1>
LEFT JOIN <table2>
ON <criteria for join>
AND <other criteria for join>
LEFT JOIN <table3>
ON <criteria for join>
AND <other criteria for join>
回答by Knowledge Craving
The required SQL will be some like:-
所需的 SQL 将类似于：-
SELECT * FROM cd
LEFT JOIN ab ON ab.sht = cd.sht
LEFT JOIN aa ON aa.sht = cd.sht
....
Hope it helps.
希望能帮助到你。
回答by RichardTheKiwi
You have two choices, depending on your table order
您有两种选择，具体取决于您的餐桌顺序
create table aa (sht int)
create table cc (sht int)
create table cd (sht int)
create table ab (sht int)
-- type 1
select * from cd
inner join cc on cd.sht = cc.sht
LEFT JOIN ab ON ab.sht = cd.sht
LEFT JOIN aa ON aa.sht = cc.sht
-- type 2
select * from cc
inner join cc on cd.sht = cc.sht
LEFT JOIN ab
LEFT JOIN aa
ON aa.sht = ab.sht
ON ab.sht = cd.sht
