One check constraint or multiple check constraints?

Question

Any suggestions on whether fewer check constraints are better, or more? How should they be grouped if at all?

Suppose I have 3 columns which are VARCHAR2(1 BYTE), each of which is a 'T'/'F' flag. I want to add a check constraint to each column specifying that only characters IN ('T', 'F') are allowed.

Should I have 3 separate check constraints, one for each column:

COL_1 IN ('T', 'F')

COL_2 IN ('T', 'F') 

COL_3 IN ('T', 'F')

Or a single check constraint:

COL_1 IN ('T', 'F') AND COL_2 IN ('T', 'F') AND COL_3 IN ('T', 'F')

My thoughts are it is best to keep these three separate, as the columns are logically unrelated to each other. The only case I would have a check constraint that examines more than one column is if there was some relationship between the value in one and the value in another, e.g.:

(PARENT_CNT > 0 AND PRIMARY_PARENT IS NOT NULL) OR (PARENT_CNT = 0 AND PRIMARY_PARENT IS NULL)

Answer 1

Keep the separate, they are different columns. Also, the error message will display the check constraint name that failed, and you will better know where the problem is. A future developer will be confused why they are all together, or not notice them since they are on a different column.

Answer 2

I recommend not using a varchar at all. This is not a standard practice for how people store booleans in databases without a boolean data type. I recommend your smallest integer type where 0 = False and non-zero = True. Constraints become trivial to check at this point (even unnecessary).

Addressing criticisms: you should make 3 constraints for debugging and maintenance reasons (better errors, logging). Performance may be slightly lessened on insert and update but no big deal.