Question

When I’m adding some constraints, e.g:

 create table Test(
  IDTest int primary key,
  Credit int not null constraint Credit check (Credit >= 0)
 );

In this case isn’t the not null in Credit redundant as I’m adding a constraint that Credit must be higher than 0?

Answer 1

No, it is not redundant.

A CHECK constraint accepts a value if the condition is not FALSE, so whether it is TRUE or UNKNOWN.

If you allow Nulls in your column, then a NULL >= 0 will evaluate to UNKNOWN and will pass the test.