What to do when I want to use database constraints but only mark as deleted instead of deleting?

Question

I am working in a project where database items are not deleted, but only marked as deleted. Something like this:

id   name     deleted
---  -------  --------         


        

Answer 1

The type of constraint you require is a table-level CHECK constraint i.e. a CHECK constraint consisting of a subquery which tests NOT EXISTS (or equivalent) for the table e.g.

CREATE TABLE Test 
(
   ID INTEGER NOT NULL UNIQUE, 
   name VARCHAR(30) NOT NULL, 
   deleted INTEGER NOT NULL, 
   CHECK (deleted IN (0, 1))
);

ALTER TABLE Test ADD
   CONSTRAINT test1__unique_non_deleted
      CHECK 
      (
         NOT EXISTS 
         (
            SELECT T1.name
              FROM Test AS T1
             WHERE T1.deleted = 0
             GROUP
                BY T1.Name
            HAVING COUNT(*) > 1
         )
      );

INSERT INTO Test (ID, name, deleted) VALUES (1, 'Thingy1', 0)
;
INSERT INTO Test (ID, name, deleted) VALUES (2, 'Thingy2', 0)
;
INSERT INTO Test (ID, name, deleted) VALUES (3, 'Thingy3', 1)
;
INSERT INTO Test (ID, name, deleted) VALUES (4, 'Thingy3', 1)
;
INSERT INTO Test (ID, name, deleted) VALUES (5, 'Thingy3', 0)
;
INSERT INTO Test (ID, name, deleted) VALUES (6, 'Thingy3', 0)
;

The last INSERT (ID = 6) will cause the constraint to bite and the INSERT will fail. Q.E.D.

...ooh, nearly forgot to mention: SQL Server doesn't yet support CHECK constraints with that contain a subquery (I tested the above on ACE/JET, a.k.a. ). While you could use a FUNCTION I've read this is unsafe due to SQL Server testing constraints on a row-by-row basis (see David Portas' Blog). Until this full SQL-92 feature is supported in SQL Server, my preferred workaround is to use the same logic in a trigger.