Question

I have a table that allows users to specify a tollerance by an absolute value or by a percentage

e.g. they could say +/- 1,000,000
or 10%

so I have a table with a column for each value

What is th best way to ensure that only one or the other gets specified, not both

I will be adding validation to my UI, but would also like this checked by the database

Some kind of custom constraint?

Answer 1

A CHECK constraint

ALTER TABLE MyTable WITH CHECK ADD
   CONSTRAINT CK_Mytable_ABSorPercent
          CHECK (ABSValue IS NULL OR PercentValue IS NULL)

This assumes your 2 columns are called ABSValue and PercentValue and both nullable. If they are nullable (they should be), change the check to your sentinel (N/A) value