本文介绍了根据主表中列的值连接表的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
notificationTable
ID type typeID userID
==============================================
1 comment 34 2
2 accept 22 2
eventTable
ID event content
===================================
21 post 34
22 accept 22
commentTable
ID comment eventID
===================================
34 'test' 21
我想根据列类型的值连接不同的表.这是我当前的伪代码:
I want to join different tables depending on the value of the column type. This is my current pseudocode:
SELECT *
FROM notificationTable notification
IF (notification.type == 'comment')
LEFT JOIN commentTable comment
ON notification.typeID = comment.ID
ELSEIF (notification.type == 'accept')
LEFT JOIN evenTable event
ON notification.typeID = event.ID
WHERE notification.userID = 2
有人知道我的意图的真正查询吗?
Anyone knows the real query for my intention?
推荐答案
类似
SELECT *
FROM notificationTable notification
LEFT JOIN commentTable comment
ON (notification.typeID = comment.ID AND notification.type == 'comment')
LEFT JOIN evenTable event
ON (notification.typeID = event.ID AND notification.type == 'accept')
WHERE notification.userID = 2
但我强烈建议重新设计你的系统以避免多态关系...
But I strongly recommend to redesign your system to avoid polymorphic relationship...
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