Multiple clauses in one check constraint?

Question

I'm trying to get multiple clauses in one check constraint, separated by an OR:

check (stop>start or stop = NULL)

where start and stop are timestamp fields. So, stop can be left empty, but if it's specified, it has to be later than start.

Am I getting this entirely wrong? I haven't found anything on timestamp comparisons or if OR is allowed in check constraints.

This is in postgres 8.4.8.

Answer 1

You should use stop IS NULL, not stop = NULL.

Answer 2

This is allowed, but:

• stop = NULL is meaningless; you mean stop IS NULL.
• there's no need to specify OR stop IS NULL anyway, because check constraints only forbid something from being "false", and stop > start will be "null" (neither "true" nor "false") if stop is null.

For more information about check constraints, see http://www.postgresql.org/docs/8.4/interactive/ddl-constraints.html.

It should be noted that a check constraint is satisfied if the check expression evaluates to true or the null value.