mysql条件连接取决于列

您不能有条件地在表之间创建联接。最好的方法是在陈述中使用

LEFT JOIN

CASE``SELECt

SELECT *,        (CASE notification_base.type WHEN 'photo' THEN photo.columnName1 ELSE post.ColumnName1        END) as ColumnName1,        (CASE notification_base.type WHEN 'photo' THEN photo.columnName2 ELSE post.ColumnName2        END) as ColumnName2FROM notification_action    INNER JOIN notification_base        ON notification_action.not_base_id = notification_base.id    INNER JOIN notification_sub        ON notification_action.not_base_id = notification_sub.not_base_id    INNER JOIN photo        ON photo.id = notification_base.object_id    INNER JOIN post        ON post.id = notification_base.object_idWHERe notification_sub.user_id = 3    AND notification_sub.lastShowDate < notification_action.creationDate;