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

For a simple table called [Test] with columns ID(int), Filter(nvarchar), Deleted(bit)

ALTER TABLE [dbo].[Test] ADD 
    CONSTRAINT [DF_Test_Deleted] DEFAULT (0) FOR [Deleted],
    CONSTRAINT [IX_Test] UNIQUE  NONCLUSTERED 
    (
        [filter],
        [Deleted]
    )  ON [PRIMARY] 

Answer 2

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.

Answer 3

Not sure about SQLServer2005, but you can define compound constrainst/indexes

CREATE UNIQUE INDEX [MyINDEX] ON [TABLENAME] ([NAME] , [DELETED])

As pointed out by SteveWeet, this will only allow you to delete/create twice.

Answer 4

It might be worth considering using a "recycle bin" table. Instead of keeping the old records in the same table with a flag, move them to its own table with its own constraints. For instance, in the active table you do have a UNIQUE constraint on name, but in the recycle bin table you don't.