Re: check with select

On Thu, 15 May 2003 edouard(dot)boucher(at)free(dot)fr wrote:

> I have a table referancing services of a company :
> CREATE TABLE Services (
> id INTEGER NOT NULL PRIMARY KEY,
> nom VARCHAR(30) NOT NULL -- name of the service
> );
>
> each of them can have many workers and many boss
> but at least 1 boss
>
> So i have a table referancing workers and boss :
>
> CREATE TABLE EmployesDsServices (
> service INTEGER REFERENCES Services (id)
> ON DELETE CASCADE DEFERRABLE INITIALLY DEFERRED,
> employe INTEGER REFERENCES Employes (id)
> ON DELETE CASCADE DEFERRABLE INITIALLY DEFERRED,
> niveau CHAR(1), -- R -> boss, S -> worker
> CONSTRAINT niveau_valide CHECK (NIVEAU IN ('R','S')),
> PRIMARY KEY (service, employe)
> );
>
> to check that at any time a service always have at least one boss,
> i would like :
>
> CREATE ASSERTION chaque_service_a_au_moins_un_responsable CHECK
> (NOT EXISTS (SELECT id FROM Services LEFT OUTER JOIN
> (SELECT * FROM employesdsservices WHERE niveau='R') AS resp ON
> id=service GROUP BY id HAVING COUNT(niveau)<1));
>
> the problem is that this is not valid in postgreSQL,

Noone's done assertions yet, and PostgreSQL also doesn't support
subqueries in check constraints at this point either AFAIK.

> so i tryed to use a trigger, but it seems that it is not possible to
> make a trigger deferrable (as the 2 tables are mutualy dependent).

It should be possible, but you have to use a not meant for general use
(and barely documented) CREATE CONSTRAINT TRIGGER, or you may be able to
twiddle the entries for a normal trigger in pg_trigger after the fact, but
I haven't tried the latter. CREATE CONSTRAINT TRIGGER was effectively
mean for references constraints in dumps before they started being dumped
as alter statements.