该行来自哪个表？答案

【问题标题】：Which table does the row come from?该行来自哪个表？
【发布时间】：2015-01-29 20:51:34
【问题描述】：

这是我的查询：

SELECT * 
FROM messages 
WHERE status = 1 
AND (
    poster IN (SELECT thing FROM follows WHERE follower = :uid AND type = 3) 
    OR 
    topic_id IN (SELECT thing FROM follows WHERE follower = :uid AND type = 1)
) 
ORDER BY post_date DESC LIMIT 0, 20

我想知道行来自哪个子句。来自poster IN (...) 部分还是topic_id IN (...) 部分？我该怎么做？

【问题讨论】：

标签： mysql sql select where-clause

【解决方案1】：

一个简单的方法：

SELECT *
     , CASE WHEN poster IN (SELECT thing FROM follows WHERE follower = :uid AND type = 3) THEN 'poster' 
            ELSE 'topic_id' END AS from_clause
FROM messages <..>

另一种方式：

SELECT m.* 
     , CASE WHEN t1.thing IS NULL THEN 'topic_id' ELSE `poster` END AS from_clause
FROM messages m 
LEFT JOIN (SELECT thing FROM follows WHERE follower = :uid AND type = 3) t1 ON m.poster = t1.thing
LEFT JOIN (SELECT thing FROM follows WHERE follower = :uid AND type = 1) t2 ON m.topic_id = t2.thing
WHERE m.status = 1  AND (t1.thing IS NOT NULL OR t2.thing IS NOT NULL)

【讨论】：

谢谢，但这不会减慢查询速度吗？让我试试。
@Wellenbrecher：嗯，它显然不会加快速度。
@Wellenbrecher ：我已经发布了另一个，还没有测试过，但试一试。