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

Per onedaywhen, this answer is criminally wrong, and an abomination. You can use a CHECK constraint. http://msdn.microsoft.com/en-us/library/ms188258.aspx

There's not a way to make conditional constraints. You should, however be able to do the job using a trigger. That's what they're for.

http://msdn.microsoft.com/en-us/library/ms189799.aspx

CREATE TRIGGER MyTable.ConditionalNullConstraint ON MyTable.ColumnB
AFTER INSERT
AS
IF EXISTS (SELECT *
    FROM inserted
    WHERE A <> 'NEW' AND B IS NULL
    )
BEGIN
    RAISERROR ('if A is ''NEW'' then B cannot be NULL', 16, 1);
    ROLLBACK TRANSACTION;
END;
GO

Note that in the query you'll want to reference inserted which is a special object that behaves like a table, and lets you reference the row(s) that caused the trigger.

Of course, in this example you'd need to handle AFTER UPDATE also to enforce the constraint, but that's the general idea.