Go Premium for a chance to win a PS4. Enter to Win

x
?
Solved

union & order by

Posted on 2013-11-17
1
Medium Priority
?
418 Views
Last Modified: 2013-11-18
Hi there,
There are 2 SELECT statements joined by UNION.
The ORDER BY comes at the end.
The question is:  Can I  also put an ORDER BY after every SELECT? If I can, does that help the performance?

eg:

SELECT * FROM (SELECT  * FROM TABLE1  ) ORDER BY ID DESC LIMIT 500 ) AS T1
UNION ALL
SELECT * FROM (SELECT  * FROM TABLE2  ) ORDER BY ID  DESC LIMIT 500 ) AS T2
ORDER BY DATETIME DESC
0
Comment
Question by:myyis
1 Comment
 
LVL 51

Accepted Solution

by:
Steve Bink earned 2000 total points
ID: 39655363
MySQL 5.5 Reference Manual :: 13.2.9.4. UNION Syntax

Most of that article pertains to the use of ORDER BY with UNION.

As a sidenote, if you want to improve performance, remove the double-SELECT from your statements:
(SELECT * FROM TABLE1 ORDER BY ID DESC LIMIT 500)
UNION ALL
(SELECT * FROM TABLE2 ORDER BY ID DESC LIMIT 500)
ORDER BY DATETIME DESC

Open in new window

To further improve performance, don't "SELECT *" - identify the columns you need and include only those columns in your field list.
0

Featured Post

Free Tool: Site Down Detector

Helpful to verify reports of your own downtime, or to double check a downed website you are trying to access.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Introduction Since I wrote the original article about Handling Date and Time in PHP and MySQL several years ago, it seemed like now was a good time to update it for object-oriented PHP.  This article does that, replacing as much as possible the pr…
In this article, we’ll look at how to deploy ProxySQL.
In this video, Percona Solution Engineer Dimitri Vanoverbeke discusses why you want to use at least three nodes in a database cluster. To discuss how Percona Consulting can help with your design and architecture needs for your database and infras…
In this video, Percona Solutions Engineer Barrett Chambers discusses some of the basic syntax differences between MySQL and MongoDB. To learn more check out our webinar on MongoDB administration for MySQL DBA: https://www.percona.com/resources/we…
Suggested Courses

886 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question