根据主表中列的值联接表

Question

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：

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？

Answer 1

就像是

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

但是我强烈建议您重新设计系统，以避免多态关系。