Solved

union & order by

Posted on 2013-11-17
1
406 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 50

Accepted Solution

by:
Steve Bink earned 500 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

VMware Disaster Recovery and Data Protection

In this expert guide, you’ll learn about the components of a Modern Data Center. You will use cases for the value-added capabilities of Veeam®, including combining backup and replication for VMware disaster recovery and using replication for data center migration.

Question has a verified solution.

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

Introduction In this article, I will by showing a nice little trick for MySQL similar to that of my previous EE Article for SQLite (http://www.sqlite.org/), A SQLite Tidbit: Quick Numbers Table Generation (http://www.experts-exchange.com/A_3570.htm…
As a database administrator, you may need to audit your table(s) to determine whether the data types are optimal for your real-world data needs.  This Article is intended to be a resource for such a task. Preface The other day, I was involved …
Microsoft Active Directory, the widely used IT infrastructure, is known for its high risk of credential theft. The best way to test your Active Directory’s vulnerabilities to pass-the-ticket, pass-the-hash, privilege escalation, and malware attacks …
Two types of users will appreciate AOMEI Backupper Pro: 1 - Those with PCIe drives (and haven't found cloning software that works on them). 2 - Those who want a fast clone of their boot drive (no re-boots needed) and it can clone your drive wh…

830 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