Can you replace or update a SQL constraint?

问题:

I have written the following constraint for a column I've called 'grade':

CONSTRAINT gradeRule CHECK grade IN (‘easy’, ‘moderate’, ‘difficult’),

Is it possible to later update the gradeRule to have different values? For example, 'moderate' and 'difficult' could be changed to 'medium' and 'hard'.

Thanks

回答1:

You could drop the existing constraint, and add the new constraint with the NOCHECK option. This would allow you to add the constraint even though data in the table violates the constraint. The problem with doing this though would be that you wouldn't be able to update existing records without making them pass the constraint first.

ALTER TABLE SomeTable DROP CONSTRAINT gradeRule
GO
ALTER TABLE SomeTable ADD CONSTRAINT gradeRule ... WITH NOCHECK
GO

Although this is possible, its not usually recommended because of the potential problems with future updates of the data.

回答2:

Drop the constraint, and then add the replacement constraint. You can't update a constraint in SQL Server at least.

ALTER TABLE SomeTable DROP CONSTRAINT gradeRule

In addition, you'll need to update the table data before adding the new constraint, so that it meets the new constraint.

回答3:

If you change the constraint, all of the data currently in the table must meet the constraint. So if you had 2 rows of data with 'moderate' and tried to change the constraint to easy, medium, and hard, it would not let you.

So you would have to make the new constraint (easy, moderate, medium, difficult, hard) or, update the data to the new values - moderate --> medium etc.