Question

Given a column with bit type in SQL Server 2008, how can I write a trigger to allow updates from 0 to 1, but disallow updating from 1 to 0?

In other words, once the bit is set to 1, it should always be 1.

The trigger must work for multiple updates, e.g. :

UPDATE Table SET BitField = 0

Should fail for any row where BitField = 1.

EDIT: To give some background, the bit/flag in question marks whether or not a monetary transaction needs to be processed. If the bit =1, the transaction has already been processed. If the bit is reset to 0, the transaction may be duplicated, so I need to enforce at the database level that the bit can not be reset to 0.

I need to protect against direct queries run against the database as well as application level bugs. I can’t be sure that a stored procedure will always be used to update the table, so I believe a trigger is the only way to enforce this logic.

Answer 1

Looks like you are in need of a simple after trigger

CREATE TABLE YourTable(
    PK int Primary key,
    bitCol bit
)

CREATE TRIGGER YourTableTrigger
   ON  YourTable
   AFTER UPDATE
AS 

    DECLARE @nrOfViolations int 

    select @nrOfViolations = count(*) from deleted  d
    join YourTable t on d.PK = t.PK
    where d.bitCol = 1 and t.bitCol = 0

    if @nrOfViolations > 0
    BEGIN
        RAISERROR('Failed', 16, 1);
        ROLLBACK TRANSACTION
    END