LIMIT within UNION?

Greetings-

I have a table of participants to be contacted for a study. Some are in
the "exposure" group, others in the "control" group. This is designated by
a column, typenr, that contains 1 for exposure, 2 for control.

The complication is this: I need to select 200 total. The 200 number
should include *all* those eligible in the exposure group, plus enough
from the control group to bring the total number up to 200. (Yes, there is
a valid reason for this.) Furthermore, I need to sort the output of the
two groups *together* by zip code.

What I've done is to write a script that counts the number of eligible
exposure candidates:

SELECT count(*) FROM participants WHERE <eligibility>AND typenr=1

Then subtract that number (currently 28) from 200 to get 172 control
participants. Then the problem starts. 

SELECT ... FROM participants
WHERE typenr=1 AND <eligibility>
UNION
SELECT ... FROM participants
WHERE typenr=2 LIMIT 172
ORDER BY zip;

returns ERROR:  parser: parse error at or near "ORDER"

I've tried a variety of parentheses to no avail.

Can someone shed some light?

Thanks!

----------------------------------------------------------------------
Andrew J Perrin - http://www.unc.edu/~aperrin
Assistant Professor of Sociology, U of North Carolina, Chapel Hill
clists@perrin.socsci.unc.edu * andrew_perrin (at) unc.edu