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From: Josh Berkus <josh@agliodbs.com>
Reply-To: josh@agliodbs.com
Organization: Aglio Database Solutions
To: Andrew Perrin <clists@perrin.socsci.unc.edu>
Subject: Re: LIMIT within UNION?
Date: Thu, 12 Sep 2002 13:20:28 -0700
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Andrew,

Another approach, one that does not require you to know before constructing=
=20
the query how many eligible subjects there are, is not to use a union at al=
l:

SELECT * FROM (
SELECT ... , zip
FROM participants
WHERE (typenr =3D 1 and <eligibility criteria)
OR (typenr =3D 2)
ORDER BY (typenr =3D 1 and <eligibility criteria>) DESC
LIMIT 200 ) p1
ORDER BY zip;

The inner query gives you all of the records that meet the eligibility=20
criteria, plus all of the records that have typenr =3D 2, in the order of=
=20
whether or not they meet the criteria (as a boolean value) and truncates it=
=20
at 200 records.
The outer query then re-sorts this result in zip order.

This seems, to me, much more flexible than using a UNION query.

--=20
-Josh Berkus
 Aglio Database Solutions
 San Francisco