Constraints and Rules Used with Null Values

You cannot define a column to allow null values, and then override this definition with a constraint or a rule that prohibits null values. For example, if a column definition specifies NULL and a rule specifies:

@val in (1,2,3)

An implicit or explicit NULL does not violate the rule. The column definition overrides the rule, even a rule that specifies:

@val is not null
Related concepts
Define Integrity Constraints for Tables
Defining Defaults and Rules for Data