Selective sequence.. - Mailing list pgsql-novice

Hello Gurus,

I would like to have a table 'members' with columns 'memberid' and
'member_role' implemented so that when I add a member it automatically
inserts a nextval('memberid') into the memberid field ONLY if
member_role='a_particular_role' otherwise 0.

I have a strong feeling that it's not too complicated at all, but I could
use a hint.

Thanks,
Timo