Comments on: Tuple Rules! Using NVL2 and NULLIF.

By: Toon Koppelaars

Toon Koppelaars — Fri, 07 Oct 2005 19:48:36 +0000

OK, ‘unequals’ should be done with ‘<&gt’, right?
alter table t add check(c1<>’I’ or c2 is not null);
and
alter table t add check(c1<>’O’ or c2 is null);

By: Toon Koppelaars

Toon Koppelaars — Fri, 07 Oct 2005 19:42:33 +0000

Hmm. Some ‘==’ disappeared. It should have read:
1) c1=â€™Iâ€™ ‘==>’ c2 is not null
and
2) c1=’O’ ‘==>’ c2 is null

By: Toon Koppelaars

Toon Koppelaars — Fri, 07 Oct 2005 19:40:17 +0000

My 2 cents on this.

I understand you actually have two tuple rules:
1) c1=’I’ ==> c2 is not null
and
2) c1=’O’ ==> c2 is null
The ‘==>’ above, is the logical implication (read as ‘implies’).

To me this would then translate into two check constraints, enabling more detailed
error messages. Using the rewrite rule for implication: “a implies b”, is logically
equivalent to “not a or b”, this then results in:

alter table t add check(c1<>‘I’ or c2 is not null);
and
alter table t add check(c1<>‘O’ or c2 is null);

Nothing fancy here…

By: Laurent Schneider

Laurent Schneider — Fri, 07 Oct 2005 15:23:50 +0000

alter table t add constraint t_chk check (c1 = ‘O’ and c2 is null or c1=’I’ and c2 is not null)
should also be possible, and imho more human readable

By: Laurent Schneider

Laurent Schneider — Fri, 07 Oct 2005 15:19:09 +0000

what do you think of
c1 = ‘O’ and c2 is null or c1=’I’ and c2 is not null

By: Andre

Andre — Fri, 07 Oct 2005 10:40:54 +0000

Nice, I just needed the same constraint on my project