CHECK Constraints vs NULL values

  • Comments posted to this topic are about the item CHECK Constraints vs NULL values

  • Good Learning for the day


  • I got it right, but on the basis that the table would be empty due to the invalid date formats used. Should have remembered the US-centric nature of this site!

  • This was removed by the editor as SPAM

  • Good question. Learned something here.

    Expected the UNKNOWN to result in a constraint violation.

  • I got it right but it was through ignorance. We don't use constraints much and I had never heard of is_not_trusted. Good info.

  • A nice one for this Friday. Thanks, Justin!

  • Nice question. Thanks Justin.

  • Good one, thanks. I got it wrong but learned something.

    - webrunner

    A SQL query walks into a bar and sees two tables. He walks up to them and asks, "Can I join you?"

  • Nice question illustrating the effect of NULLs on CHECK constraints, which I suspect confuses many people. Not surprising that 60% of answers so far have it wrong.

    Looking for FALSE and treating UNKOWN as TRUE in a CHECK constraint is sort of reasonable because if one looks for TRUE and treats UNKNOWN as false the CHECK constraint effectively enforces a NOT NULL constraint on each column referenced in the CHECK constraint and there's already a perfectly good way of doing that (specifying NOT NULL in the column definitions). Effectively treating UNKNOWN as TRUE is the only viable way of making CHECK constraints apply to nullable columns. But it bewildered me the first time I came across it until I had done some hard thinking.


  • good thoughtful question. really nice share. Thanks

  • Great question, thanks.

    Need an answer? No, you need a question
    My blog at
    MCSE Business Intelligence - Microsoft Data Platform MVP

  • So is this what is needed?

    ALTER TABLE dbo.Test ADD CONSTRAINT CK_Test_EndDateAfterStartDateAndNotNull CHECK (EndDate > StartDate AND EndDate IS NOT NULL);


    well, I could also do this:

    CREATE TABLE dbo.Test




    EndDate DATETIME not NULL



    ALTER TABLE dbo.Test ADD CONSTRAINT CK_Test_EndDateAfterStartDate CHECK (EndDate > StartDate);


    I normally default Enddate to "9999-12-31", and do not allow Unknown dates.

  • Thanks for the question.

Viewing 15 posts - 1 through 15 (of 20 total)

You must be logged in to reply to this topic. Login to reply