mysql中的条件连接

conditional join in mysql(mysql中的条件连接)

问题描述

我有一个表 id1、id2、type.type 是一个枚举值，包含另一个表的名称.我想使用 type 表的名称执行 join.例如:

I have a table id1, id2, type. type is an enumerated value containing a name of another table. I'd like to preform a join with the name of the table of type. For example:

switch($type)
case 'table1':
   join table1;
   break;
case 'table2':
   join table2;
   break;

我怎样才能做到这一点?

How can I achieve this?

推荐答案

你不能直接那样做……你可以这样做(不是很漂亮……):

You can't do it directly like that... you can do something like this though (not very pretty...):

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'

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