The parens are for making sure the parsing works correctly. Probably either 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
>
> >>>> 2012/06/20 14:32 -0700, Rick James >>>>
> (
> SELECT ruid1, ruid2, overlap FROM l4_link WHERE ruid1=xxx
> UNION
> SELECT ruid1, ruid2, overlap FROM l4_link WHERE ruid2=xxx
> ) ORDER BY overlap DESC;
>
> 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=xxx)
> UNION
> (SELECT ruid1, ruid2, overlap FROM l4_link WHERE ruid2=xxx)
> ORDER BY overlap DESC
> but bracketing both is not mentioned. What is the difference?
>
>
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