GROUP BY / HAVING - am I being thick?

I have a question about GROUP BY / HAVING. It seems I can't do
something I *think* I should be able to do!


Give the following table called 'summary':
------------------------
(int)   (bool) (int)
codon   expl   nexamples
------------------------286     t        1286     f        1286     f        4286     t       13286     t       37286
 t        1287     f        3287     f        3287     f        4287     f        1288     f        2288     f
3288    f        4288     f        2289     f        1289     t        3
 
------------------------

For each codon number, I want to obtain sum(nexamples) where expl = 'f';

i.e. I want to get the result:
------------------------
(int)   (int)
codon   sum
------------------------286     5287     11288     11289     1
------------------------

I can do this using a temporary table (or equally using a view):       SELECT codon, nexamples INTO summary_tmp
FROMsummary       WHERE expl = 'f';       SELECT codon, sum(nexamples)       FROM summary_tmp       GROUP BY codon;
 DROP TABLE summary_tmp; 
 
but I thought I should be able to do it using GROUP BY and HAVING.
The following looks as though it should work according to "The
Practical SQL Handbook" (Bowman, Emerson & Darnovsky)

SELECT codon, sum(nexamples)
FROM summary
WHERE expl = 'f'
GROUP BY codon
HAVING expl = 'f';

I've tried various contortions of putting expl in the SELECT clause
and in the GROUP BY clause, but nothing seems to work. The docs say that
PostgreSQL requires the HAVING clause "unambiguously reference a grouping
column" whatever that means - I tried putting expl in the GROUP BY, but it
didn't help.


Any suggestions gratefully received!


Thanks,

Andrew



--
Dr. Andrew C.R. Martin          EMail:  a.c.r.martin@reading.ac.uk  (work)
Lecturer in Bioinformatics              andrew@stagleys.demon.co.uk (home)
University of Reading
Tel.: +44 (0)118 987 5123x7022  Fax:    +44 (0)118 931 0180