SQL 3表sql连接
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3 table sql join
提问by Sam
I need to join tableA, tableB, and tableC, but it's possible that tableB won't have a corresponding row. Posted below is how I am currently doing the query. The problem with it is that if tableB doesn't have a correspoinding row, it won't return a result. My sql skills are very rusty so I appreciate your help. Thanks.
我需要连接 tableA、tableB 和 tableC，但 tableB 可能没有相应的行。下面发布的是我目前如何进行查询。它的问题在于，如果 tableB 没有相应的行，则不会返回结果。我的 sql 技能非常生疏，所以我感谢你的帮助。谢谢。
SELECT [column names]
FROM tableA AS a, tableB AS b, tableC as c
WHERE b.blah = a.blah
AND c.foo = a.foo
AND [more where conditions]
回答by MatBailie
Don't use the ,syntax. Use JOINto allow for a readable LEFT JOIN...
不要使用,语法。使用JOIN以允许读取LEFT JOIN...
SELECT
*
FROM
tableA
LEFT JOIN
tableB
ON TableB.x = TableA.y
LEFT JOIN
tableC
ON TableC.x = TableB.y
AND TableC.y = TableA.z
回答by Michael Fredrickson
Use a LEFT JOIN.
使用一个LEFT JOIN.
SELECT [column names]
FROM
tableA AS a
LEFT JOIN tableB AS b ON b.blah = a.blah
JOIN tableC as c ON c.foo = a.foo
回答by Mark Sherretta
SELECT [column names]
FROM tableA AS a INNER JOIN tableC as c ON (c.foo = a.foo)
LEFT OUTER JOIN tableB as B on (b.blah = a.blah)
WHERE [more where conditions]
If the [more where conditions] are on B, then you need to include them in the OUTER JOIN ON clause.
如果 [more where conditions] 在 B 上，那么您需要将它们包含在 OUTER JOIN ON 子句中。
回答by Chris Cunningham
The terminology here (so you can look up more details) is an Outer Join. The other answer(s) are fine -- but watch out for whether you want an INNERor OUTERjoin on table c (do you want records returned if there are no matching rows in tableC)? Here is a link for the general issue.
此处的术语（以便您可以查找更多详细信息）是外部联接。其他答案很好——但是要注意您是否想要表 c 上的anINNER或OUTERjoin （如果 tableC 中没有匹配的行，您是否希望返回记录）？这是一般问题的链接。
