subselects in check clauses

I have a need to check if a certain condition holds for all rows that
matches a given criteria. But postgresql appearently doesn't support
subselects in check clauses. Here is an example:

CREATE TABLE example (a int, b date, c date
CONSTRAINT csname CHECK (1 >= ALL (SELECT count(*)
                                   FROM example
                           WHERE age(now(),b) > '0:0'
                                   AND age(now(),c) < '0:0'
                           GROUP BY a)));

CREATE

INSERT INTO example VALUES (0,'2001-3-01','2001-12-12');
ERROR:  ExecEvalExpr: unknown expression type 108

What can I do?
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Rasmus Resen Amossen | stud.mat.dat at the University of Copenhagen
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