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
See “Defining integrity constraints for tables” for more information on constraints. Rules are covered in detail in Chapter 13, “Defining Defaults and Rules for Data.”
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