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I've got a problem that I can't quite wrap my head around, about adding
a constraint to my PostgreSQL genealogy database. If somebody are
interested, I've written some thoughts on the general design at
<http://solumslekt.org/forays/blue.php>.
I've got two tables, persons and relations. I need a separate relations
table for source referencing and discussion. Here are my preliminary
definitions (irrelevant columns removed):
CREATE TABLE persons ( person_id INTEGER PRIMARY KEY, gender SMALLINT NOT NULL DEFAULT 0 CHECK
(genderIN (0,1,2,9)) -- ISO gender codes
);
CREATE TABLE relations ( relation_id INTEGER PRIMARY KEY, child_fk INTEGER REFERENCES persons (person_id),
parent_fkINTEGER REFERENCES persons (person_id), CONSTRAINT child_parent UNIQUE (child_fk, parent_fk)
);
Now, I want to ensure that each person_id can be assigned only one
father (gender=1) and one mother (gender=2). (Yes, this is old-
fashioned, but I'm working with 18th century people). How do I do it?
I have tried this:
ALTER TABLE relations ADD CONSTRAINT non_unique_father CHECK (NOT EXISTS (SELECT persons.person_id,
relations.parent_fk FROM persons AS P, relations AS R WHERE R.parent_fk = P.person_id AND
P.gender= 1));
But psql replies with:
pgslekt=> \i install/add_unique_father_and_mother_constraint.sql
psql:install/add_unique_father_and_mother_constraint.sql:9: NOTICE:
adding missing FROM-clause entry in subquery for table "persons"
psql:install/add_unique_father_and_mother_constraint.sql:9: ERROR:
cannot use subquery in check constraint
From what I've found on Google, it looks like the "cannot use subquery
in check constraint" is a real limitation in PostgreSQL. Can I use a
trigger to achieve what I want? I'm still a little shaky on triggers
and what they can do, having quite recently converted to PostgreSQL
from a certain Swedish dinky-db.
--
Leif Biberg Kristensen
http://solumslekt.org/