Question

I’m trying to create a check on a few columns in my database that forces them to be null unless the value of another column is yes. I’ve tried something like

ALTER TABLE TABLE1
ALTER Column1
CHECK (Column2 = 'y')

But that doesn’t seem to be working. Is there a way to do it like this, or is there a better way around this problem? Any help in the right direction would be great.

Answer 1

If you want to require that col1 is null unless col2 is ‘y’, you can write your CHECK constraint as:

col2 = 'y' OR col1 IS NULL

If you additionally want to require that col1 must have a value when col2 is ‘y’, you can write the constraint as:

(col2 = 'y' AND col1 IS NOT NULL) OR (col2 != 'y' AND col1 IS NULL)

You should write this as a table constraint, btw. I don’t think a column constraint is allowed to refer to other columns. (But a column constraint is just another way of writing a table constraint anyway, so you don’t lose anything by writing table constraints instead.)