From: Rick James
Date: June 22 2012 12:58am
Subject: RE: UNION and ORDER BY
List-Archive: http://lists.mysql.com/mysql/227728
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The parens are for making sure the parsing works correctly.  Probably eithe=
r one works fine.  Suggest you do
  EXPLAIN EXTENDED ...;
  SHOW WARNINGS;
I suspect that the output from each will be identical, and have more parens=
.

The main need for parens is to avoid associating the ORDER BY with just the=
 second SELECT.

> -----Original Message-----
> From: Hal?sz S?ndor [mailto:hsv@stripped]
> Sent: Thursday, June 21, 2012 4:07 PM
> To: mysql@stripped
> Subject: UNION and ORDER BY
>=20
> >>>> 2012/06/20 14:32 -0700, Rick James >>>>
> (
>    SELECT ruid1, ruid2, overlap FROM l4_link WHERE ruid1=3Dxxx
>    UNION
>    SELECT ruid1, ruid2, overlap FROM l4_link WHERE ruid2=3Dxxx
> ) ORDER BY overlap DESC;
>=20
> Make it UNION ALL or UNION DISTINCT depending on whether xxx can be in
> both fields of one row.  UNION DISTINCT makes a pass over the temp
> table to dedup.
> <<<<<<<<
> The help example shows a UNION s each SELECT separatly round-bracketed,
>     (SELECT ruid1, ruid2, overlap FROM l4_link WHERE ruid1=3Dxxx)
>     UNION
>     (SELECT ruid1, ruid2, overlap FROM l4_link WHERE ruid2=3Dxxx)
>   ORDER BY overlap DESC
> but bracketing both is not mentioned. What is the difference?
>=20
>=20
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