Question

我正在尝试使用工会从不同的表中选择项目，按日期排序（最新的第一个）

说我有以下表格

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，但我从订单中得到错误

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

Answer 1

正如UNION Syntax所述：

要使用ORDER BY或LIMIT子句对整个UNION结果进行排序或限制，请为各个SELECT语句添加括号并放置ORDER BY或{ {1}}在最后一个之后。以下示例使用两个子句： LIMIT

因此：

(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;

在sqlfiddle上查看。

Answer 2

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

Answer 3

你只需要一个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 ;

使用mysql选择和排序

3 个答案: