使用mysql联合选择和排序答案

【问题标题】：union select and order by with mysql使用mysql联合选择和排序
【发布时间】：2012-08-27 01:19:16
【问题描述】：

我正在尝试使用联合从不同表中选择项目，按日期排序（最新的优先）

假设我有以下表格

events
eventsID  | eventsTitle      | eventsTimestamp  |  eventsLocation
 1          event title         2012/08/23            1
 2          event title 2       2012/08/15            1


posts
postsID | postsSubject | postsTimestamp | postsCategory
 1          post title     2012/08/20         1

所以输出应该是

title           timestamp
event Title       2012/08/23 
post title        2012/08/20
event title 2     2012/08/15

这是我正在尝试做的事情，但我从 order by 中得到一个错误

SELECT posts.postsID, posts.postsSubject FROM posts where posts.postsCategory = 1 ORDER BY posts.postsTimestamp DESC
UNION
SELECT events.eventsID, events.eventsTitle FROM events where events.eventsLocation = 1 ORDER BY events.eventsTimestamp DESC

【问题讨论】：

标签： mysql sql-order-by union

【解决方案1】：

如UNION Syntax下所述：

要使用ORDER BY 或LIMIT 子句对整个UNION 结果进行排序或限制，请将各个SELECT 语句括起来并将ORDER BY 或LIMIT 放在最后一个语句之后。以下示例同时使用了这两个子句：
(SELECT a FROM t1 WHERE a=10 AND B=1)
UNION
(SELECT a FROM t2 WHERE a=11 AND B=2)
ORDER BY a LIMIT 10;

因此：

 (SELECT postsSubject AS title, postsTimestamp AS timestamp
  FROM   posts
  WHERE  postsCategory = 1)

UNION ALL

 (SELECT eventsTitle, eventsTimestamp
  FROM   events
  WHERE  eventsLocation = 1)

ORDER BY timestamp DESC

在sqlfiddle 上查看。

【讨论】：

查询执行后有没有办法区分post和event？例如一个是 event.php?id=1 而另一个可能是 post.php?id=1
@user1104854：是的，您可以在每个 SELECT 中包含一个新列 - 例如(SELECT 'post' AS type, postsSubject ...) UNION ALL (SELECT 'event', eventsTitle ...) ORDER BY timestamp DESC.

【解决方案2】：

您只需要一个ORDER BY 子句，最后，在 2 个联合选择之后：

SELECT postsID          AS id, 
       postsSubject     AS title, 
       postsTimestamp   AS timestamp
FROM posts 
WHERE postsCategory = 1 

UNION

SELECT eventsID, 
       eventsTitle, 
       eventsTimestamp 
FROM events 
WHERE eventsLocation = 1 

ORDER BY timestamp DESC ;

【讨论】：

事件部分的选择语句是否需要将 eventsTimestamp 定义为某种东西？
不，第二个 SELECT 列表不需要别名。使用第一个 SELECT 列表中的那些。
假设我想添加另一个表（我们称之为tableB）。我是否只需在事件选择语句之后添加 tableB 而不用 AS 语句修改帖子部分中的任何内容？

【解决方案3】：

试试这个，

SELECT Title, `TimeStamp`
(
    SELECT  posts.postsID as ID, 
            posts.postsSubject as Title, 
            eventsTimestamp as `TimeStamp`
    FROM    posts 
    where   posts.postsCategory = 1 
    UNION
    SELECT  events.eventsID as ID, 
            events.eventsTitle as Title, 
            postsTimestamp as `TimeStamp`
    FROM    events 
    where   events.eventsLocation = 1 
) x
ORDER BY `TimeStamp` DESC

【讨论】：