Sql Conditional Not Null Constraint

Question

I am curious to know is it possible to create a conditional not null constraint in sql? In otherwords is it possible to create a constraint such that a column B can be null

Answer 1

I think your first stated requirement is:

IF ( B IS NULL ) THEN ( A = 'NEW' )

Apply the implication rewrite rule:

IF ( X ) THEN ( Y )   <=>   ( NOT ( X ) OR ( Y ) )

In your case;

( NOT ( B IS NULL ) OR ( A = 'NEW' ) )

Minor rewrite to take advantage of SQL syntax:

( B IS NOT NULL OR A = 'NEW' )

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Your second stated ("extend") requirement:

IF ( A = 'NEW' ) THEN ( B IS NULL )

Apply rewrite rule:

( NOT ( A = 'NEW' ) OR ( B IS NULL ) )

Minor rewrite:

( A <> 'NEW' OR B IS NULL )