Re: Constraint allowing value up to 2 times but no more than 2 times

On 10/12/15 12:04 PM, Dara Unglaube wrote:
> Greeetings.
> I'm trying to come up with a way to allow one column to have the same
> value up to two times, but no more than two times. I attempted adding a
> constraint to check the count of the value in the field  - count
> (trsqqq) <=2 but aggregate functions are not allowed in constraints. Is
> there another way to do this? Any help would be greatly appreciated.

The safest bet is some kind of unique constraint. That would require
having a separate count field, which you would limit to being 1 or 2 via
a CHECK constraint. You could use a trigger to set the value based on
what's already in the table.

By the way, the issue with doing a simple count is that it's not safe
from race conditions, like an insert and a delete happening together.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com