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9.1

MySQL Union with Order By and Limit

Asked by ncoo in MySQL Server, PHP and Databases, PL / SQL

Tags: MySQL, Union, Order By, Limit, SQL

If I have two select statements and use union to join them where is the best place to put order by and limit.

For example:

( select * AS alias_names FROM T1 )
UNION
( select * AS alias_names FROM T2 )
ORDER BY alias_id DESC
LIMIT BY 1,11

or would this be better:

( select * AS alias_names FROM T1 ORDER BY T1_id DESC LIMIT BY 1,11)
UNION
( select * AS alias_names FROM T2 ORDER BY T2_id DESC LIMIT BY 1,11)
ORDER BY alias_id DESC
LIMIT BY 1,11

I know alias names can not be used like that.

I'm interested in the performance issues of both queries, which query is faster?

My thoughts are Query 1 is faster up to X records and then Query 2 is faster, but what is X?

I have looked at the MySQL Reference Manual it seems to suggest this, can anyone provide additional information about this and experience with it?
 
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[+][-]04/12/09 04:09 AM, ID: 24125128Accepted Solution

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About this solution

Zones: MySQL Server, PHP and Databases, PL / SQL
Tags: MySQL, Union, Order By, Limit, SQL
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Solution Provided By: rrjegan17
Participating Experts: 2
Solution Grade: A
 
[+][-]04/12/09 06:48 AM, ID: 24125697Author Comment

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[+][-]04/12/09 07:56 AM, ID: 24125872Expert Comment

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[+][-]04/12/09 09:44 AM, ID: 24126169Author Comment

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[+][-]04/12/09 09:59 AM, ID: 24126213Assisted Solution

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