Thread: Two tables refenceing each other's columns
Is something like the following allowed (or is not a Bad Idea)? table1 ---------+------------------------- id1 |serial primary key col2 |int references table2(id2) table2 ---------+------------------------- id2 |serial primary key col2 |int references table1(id1) Obviously, creating the tables is a problem since the constraints require that the other table exists. If doing the above is *not* a Bad Idea, how could I work around this problem? (That is, (how) can I add the constraints after table creation? I imagine something with "create constraint trigger", but the manual is not very clear on that.) Thanks dan
Here is some code I played with before. It does what you want. Just make a
new database to try it in.
-- Load the PGSQL procedural language
-- This could also be done with the createlang script/program.
-- See man createlang.
CREATE FUNCTION plpgsql_call_handler()
RETURNS OPAQUE AS '/usr/local/pgsql/lib/plpgsql.so'
LANGUAGE 'C';
CREATE TRUSTED PROCEDURAL LANGUAGE 'plpgsql'
HANDLER plpgsql_call_handler
LANCOMPILER 'PL/pgSQL';
CREATE TABLE test1 (id INTEGER PRIMARY KEY);
CREATE TABLE test2 (id INTEGER PRIMARY KEY);
ALTER TABLE test1 ADD CONSTRAINT test1_id_fk
FOREIGN KEY (id) REFERENCES test2
ON UPDATE CASCADE
ON DELETE CASCADE -- can delete test2
INITIALLY DEFERRED;
ALTER TABLE test2 ADD CONSTRAINT test1_id_fk
FOREIGN KEY (id) REFERENCES test1
ON UPDATE CASCADE
ON DELETE RESTRICT -- disallows delete test1
INITIALLY DEFERRED;
CREATE SEQUENCE test_id_seq;
CREATE FUNCTION new_tests() RETURNS INTEGER AS '
DECLARE
new_seq INTEGER;
BEGIN
new_seq := nextval(''test_id_seq'');
INSERT INTO test1 VALUES (new_seq);
INSERT INTO test2 VALUES (new_seq);
RETURN new_seq;
END;
' LANGUAGE 'plpgsql';
-- implicit BEGIN;
SELECT new_tests();
-- implicit COMMIT;
SELECT new_tests();
SELECT new_tests();
SELECT * FROM test1;
SELECT * FROM test2;
DELETE FROM test1 WHERE id = 1; -- this will fail
DELETE FROM test2 WHERE id = 1; -- this will succeed and cacade
SELECT * FROM test1;
SELECT * FROM test2;
On Tuesday 02 January 2001 01:57, GH wrote:
> Is something like the following allowed (or is not a Bad Idea)?
>
> table1
> ---------+-------------------------
> id1 |serial primary key
> col2 |int references table2(id2)
>
> table2
> ---------+-------------------------
> id2 |serial primary key
> col2 |int references table1(id1)
>
>
> Obviously, creating the tables is a problem since the constraints require
> that the other table exists.
> If doing the above is *not* a Bad Idea, how could I work around this
> problem?
> (That is, (how) can I add the constraints after table creation?
> I imagine something with "create constraint trigger", but the manual is
> not very clear on that.)
>
>
> Thanks
>
> dan
--
-------- Robert B. Easter reaster@comptechnews.com ---------
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On Tue, Jan 02, 2001 at 02:27:28AM -0500, some SMTP stream spewed forth:
> Here is some code I played with before. It does what you want. Just make a
> new database to try it in.
Great, thanks.
I ended up working around it by storing one of the primary keys
in another table with some other information (which is a better way
to have the tables setup, in my case).
I am glad to have your code on hand though.
Thanks
dan
>
>
>
> -- Load the PGSQL procedural language
> -- This could also be done with the createlang script/program.
> -- See man createlang.
> CREATE FUNCTION plpgsql_call_handler()
> RETURNS OPAQUE AS '/usr/local/pgsql/lib/plpgsql.so'
> LANGUAGE 'C';
> CREATE TRUSTED PROCEDURAL LANGUAGE 'plpgsql'
> HANDLER plpgsql_call_handler
> LANCOMPILER 'PL/pgSQL';
>
> CREATE TABLE test1 (id INTEGER PRIMARY KEY);
> CREATE TABLE test2 (id INTEGER PRIMARY KEY);
>
> ALTER TABLE test1 ADD CONSTRAINT test1_id_fk
> FOREIGN KEY (id) REFERENCES test2
> ON UPDATE CASCADE
> ON DELETE CASCADE -- can delete test2
> INITIALLY DEFERRED;
> ALTER TABLE test2 ADD CONSTRAINT test1_id_fk
> FOREIGN KEY (id) REFERENCES test1
> ON UPDATE CASCADE
> ON DELETE RESTRICT -- disallows delete test1
> INITIALLY DEFERRED;
>
> CREATE SEQUENCE test_id_seq;
>
> CREATE FUNCTION new_tests() RETURNS INTEGER AS '
> DECLARE
> new_seq INTEGER;
> BEGIN
> new_seq := nextval(''test_id_seq'');
> INSERT INTO test1 VALUES (new_seq);
> INSERT INTO test2 VALUES (new_seq);
> RETURN new_seq;
> END;
> ' LANGUAGE 'plpgsql';
>
> -- implicit BEGIN;
> SELECT new_tests();
> -- implicit COMMIT;
> SELECT new_tests();
> SELECT new_tests();
>
> SELECT * FROM test1;
> SELECT * FROM test2;
>
> DELETE FROM test1 WHERE id = 1; -- this will fail
> DELETE FROM test2 WHERE id = 1; -- this will succeed and cacade
>
> SELECT * FROM test1;
> SELECT * FROM test2;
>
>
>
>
> On Tuesday 02 January 2001 01:57, GH wrote:
> > Is something like the following allowed (or is not a Bad Idea)?
> >
> > table1
> > ---------+-------------------------
> > id1 |serial primary key
> > col2 |int references table2(id2)
> >
> > table2
> > ---------+-------------------------
> > id2 |serial primary key
> > col2 |int references table1(id1)
> >
> >
> > Obviously, creating the tables is a problem since the constraints require
> > that the other table exists.
> > If doing the above is *not* a Bad Idea, how could I work around this
> > problem?
> > (That is, (how) can I add the constraints after table creation?
> > I imagine something with "create constraint trigger", but the manual is
> > not very clear on that.)
> >
> >
> > Thanks
> >
> > dan
>
> --
> -------- Robert B. Easter reaster@comptechnews.com ---------
> - CompTechNews Message Board http://www.comptechnews.com/ -
> - CompTechServ Tech Services http://www.comptechserv.com/ -
> ---------- http://www.comptechnews.com/~reaster/ ------------
You can use ALTER TABLE ADD CONSTRAINT to add foreign key constraints after table creation. On Tue, 2 Jan 2001, GH wrote: > > Is something like the following allowed (or is not a Bad Idea)? > > table1 > ---------+------------------------- > id1 |serial primary key > col2 |int references table2(id2) > > table2 > ---------+------------------------- > id2 |serial primary key > col2 |int references table1(id1) > > > Obviously, creating the tables is a problem since the constraints require > that the other table exists. > If doing the above is *not* a Bad Idea, how could I work around this > problem? > (That is, (how) can I add the constraints after table creation? > I imagine something with "create constraint trigger", but the manual is > not very clear on that.) > > > Thanks > > dan >