April 30, 2015 at 11:55 pm
Comments posted to this topic are about the item CHECK Constraints vs NULL values
April 30, 2015 at 11:59 pm
Good Learning for the day
Thanks
May 1, 2015 at 2:31 am
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!
May 1, 2015 at 4:08 am
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May 1, 2015 at 5:21 am
An easy way to end the week. Thanks.
May 1, 2015 at 5:26 am
Good question. Learned something here.
Expected the UNKNOWN to result in a constraint violation.
May 1, 2015 at 9:14 am
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.
May 1, 2015 at 10:18 am
A nice one for this Friday. Thanks, Justin!
May 1, 2015 at 11:05 am
Nice question. Thanks Justin.
May 1, 2015 at 3:40 pm
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?"
Ref.: http://tkyte.blogspot.com/2009/02/sql-joke.html
May 1, 2015 at 5:22 pm
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.
Tom
May 4, 2015 at 12:02 am
good thoughtful question. really nice share. Thanks
May 4, 2015 at 1:54 am
Great question, thanks.
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May 5, 2015 at 5:02 am
So is this what is needed?
ALTER TABLE dbo.Test ADD CONSTRAINT CK_Test_EndDateAfterStartDateAndNotNull CHECK (EndDate > StartDate AND EndDate IS NOT NULL);
GO
well, I could also do this:
CREATE TABLE dbo.Test
(
ID INT NOT NULL,
StartDate DATETIME NOT NULL,
EndDate DATETIME not NULL
);
GO
ALTER TABLE dbo.Test ADD CONSTRAINT CK_Test_EndDateAfterStartDate CHECK (EndDate > StartDate);
GO
I normally default Enddate to "9999-12-31", and do not allow Unknown dates.
May 5, 2015 at 6:54 am
Thanks for the question.
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