Re: [SQL] Auto increment field when updating?

Robin Keech <robin@dialogue.co.uk> writes:
> ... Only some fields are written to each time the row is updated.
> There is one field called 'count' that ONLY needs to increment every
> time that particular row AND the field 'count' are updated.

> I know that I can select the information, increment it and update it
> back again, but is there any way of setting up the database to do this
> automatically?  I am using PostgreSQL 6.5.

Sure, use a trigger.  Here's one I use for a table that wants to attach
a globally new sequence number to each created or updated row:

CREATE FUNCTION my_phistory_trigger() RETURNS OPAQUE AS '
BEGIN NEW.sequenceNo := nextval(''my_phistory_Seq''); RETURN NEW;
END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER my_phistory_trigger BEFORE INSERT OR UPDATE ON my_phistory
FOR EACH ROW EXECUTE PROCEDURE my_phistory_trigger();

but if you just want to increment the number that's there, you could do
 NEW.sequenceNo := OLD.sequenceNo + 1;

(bearing in mind that this'll only work as an UPDATE trigger, not
INSERT).  See the programmer's guide for doco on trigger functions.
        regards, tom lane