mysql条件连接取决于列

Question

我正在尝试实现通用通知系统..我有这样的数据结构;

notification_base:id,type,object
notificiation_sub:id,user_id,not_base_id,lastNotifyTime
notification_action:id,user_id,not_base_id,action,creationDate

所以基本情况是,用户通过发布状态或上传照片等创建通知基础项目.(在这种情况下,照片指的是notification_base表中的类型字段,object_id是post_id或photo_id取决于类型)然后用户订阅对于此notification_base项目..(用户3订阅notification_base 5和最后通知时间x)

之后,另一个用户触摸此notification_base项.(例如,评论状态或类似照片)此操作记录在notification_action表中(用户5在12/02/2011上执行'动作').

我想要的是从用户订阅中获取notification_base项,如果最后notifiytime小于通知操作然后将它们与notification_action连接.我可以使用此sql成功;

对于用户ID 3;

select * 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
                where notification_sub.user_id = 3 and notification_sub.lastShowDate < notification_action.creationDate ;

结果几乎就是我想要的,例如

user x对你的对象做了'action',它在时间t有'type'和object_id

但我也想加入object_id取决于类型..所以我实际上可以学习触摸哪个对象..但是你可以看到类型是动态的,如果type = post对象id指的是post表上的post_id,如果type = photo object id在照片桌等上引用photo_id ..

我尝试做这样的事情,但得到一些语法错误;

SELECT *
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 CASE notification_base.type
            WHEN 'photo'
                THEN (
                        INNER JOIN photo
                            ON photo.id = notification_base.object_id
                        )
            ELSE (
                    INNER JOIN post
                        ON post.id = notification_base.object_id
                    )
            END
WHERE notification_sub.user_id = 3
    AND notification_sub.lastShowDate < notification_action.creationDate;

我知道这不正确,就像伪代码一样

Answer 1

您不能有条件地创建跨表的连接.最好的方法是在你的陈述中使用LEFT JOIN和使用你.您必须指定要显示的公共列,CASESELECT

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;