mysql 中的条件连接
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conditional join in mysql
提问by Greg
I have a table id1, id2, type. typeis an enumerated value containing a name of another table.
I'd like to preform a joinwith the name of the table of type.
For example:
我有一个表id1，id2，type。type是包含另一个表名称的枚举值。我想join用type. 例如：
switch($type)
case 'table1':
join table1;
break;
case 'table2':
join table2;
break;
How can I achieve this?
我怎样才能做到这一点？
回答by Greg
You can't do it directly like that... you can do something like this though (not very pretty...):
你不能直接那样做......你可以做这样的事情（不是很漂亮......）：
SELECT
t.id,
t.type,
t2.id AS id2,
t3.id AS id3
FROM t
LEFT JOIN t2 ON t2.id = t.id AND t.type = 't2'
LEFT JOIN t3 ON t3.id = t.id AND t.type = 't3'
回答by Steve B.
ugly way:
丑陋的方式：
Table Types, T1, T2:
表类型，T1，T2：
SELECT ... FROM Types, T1 , where Types.ID=T1.Types_ID AND Types.TYPE='TABLE1'
UNION
SELECT ... FROM Types, T2 , where Types.ID=T2.Types_ID AND Types.TYPE='TABLE2'
回答by Ilan
In addition to previous answer: You can combine the two left joins results by using IF statement: IF(t.type= 't2', t2.id, t3.id) as type_id
除了先前的答案：您可以使用 IF 语句组合两个左连接结果： IF(t.type='t2', t2.id, t3.id) as type_id
You can see another mysql conditional joinsexample at mysqldiary.com
您可以在 mysqldiary.com 上看到另一个mysql 条件连接示例
