Andrew,
Another approach, one that does not require you to know before constructing
the query how many eligible subjects there are, is not to use a union at all:
SELECT * FROM (
SELECT ... , zip
FROM participants
WHERE (typenr = 1 and <eligibility criteria)
OR (typenr = 2)
ORDER BY (typenr = 1 and <eligibility criteria>) DESC
LIMIT 200 ) p1
ORDER BY zip;
The inner query gives you all of the records that meet the eligibility
criteria, plus all of the records that have typenr = 2, in the order of
whether or not they meet the criteria (as a boolean value) and truncates it
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.
--
-Josh BerkusAglio Database SolutionsSan Francisco
