Re: unique value - trigger?

n Thu, Jul 17, 2003 at 12:56:52PM +0100, Gary Stainburn wrote:
>
> nymr=# \d lnumbers
>                Table "lnumbers"
>   Column   |         Type          | Modifiers
> -----------+-----------------------+-----------
>  lnid      | integer               | not null
>  lnumber   | character varying(10) | not null
>  lncurrent | boolean               |
> Primary key: lnumbers_pkey
> Triggers: RI_ConstraintTrigger_7575462
>
> I want to make it so that if I set lncurrent to true for one row, any existing
> true rows are set to false.
>
> I'm guessing that I need to create a trigger to be actioned after an insert or
> update which would update set lncurrent=false where lnid not = <current lnid> 

Absolutely. Something like this will work:

CREATE FUNCTION lnumbers_current_trigger() RETURNS TRIGGER AS '
BEGIN IF NEW.lncurrent THEN   UPDATE lnumbers SET lncurrent = ''f''   WHERE lnid = NEW.lnid AND lnumber != NEW.lnumber
ANDlncurrent = ''t''; END IF; RETURN NEW;
 
END' LANGUAGE 'plpgsql';

CREATE TRIGGER lnumbers_current AFTER UPDATE OR INSERT ON lnumbers
FOR EACH ROW EXECUTE PROCEDURE lnumbers_current_trigger();

(Lightly tested only on 7.3.3)

In the WHERE condition in the function, specifying "lncurrent = 't'"
means that we don't update more rows than we have to. Making the
trigger fire AFTER INSERT as well as AFTER UPDATE means that if you
just add a new row with lncurrent set to true, it Does The Right
Thing. In this particular example, the trigger will work perfectly
well as a BEFORE, also.

If you can't or don't want to install PL/PgSQL (or some other
procedural language), you can do it with rules. It's more long-winded
that way, although I also think it's more elegant...


Richard