mysql conditional join depends on a column

Question

I\'m trying to implement a generic notification system.. I have a data structure like this;

notification_base:id,type,object
notificiation_sub:id,user_id,not         


        

Answer 1

you can't conditionally create joins across tables. The best way to do is to use LEFT JOIN and use you CASE in your SELECT statement. You must have specify common columns you want to show,

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 ColumnName2
FROM 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_id
WHERE notification_sub.user_id = 3
    AND notification_sub.lastShowDate < notification_action.creationDate;