SQL trigger to prevent bit column from being updated from 1 to 0? - sql

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

Answer 2

I wouldn't bury it in a Trigger. I'd have your stored procedure (SP) that updates the table do a check on the value. For example:
CREATE PROCEDURE dbo.proc_update_my_table
#id AS INT,
-- Whatever other params you need
#the_bit_field AS BIT
AS
BEGIN
SET NOCOUNT ON;
DECLARE #existing_value AS BIT
SELECT #existing_value = the_bit_field FROM dbo.Table1 t WHERE t.id = #id
IF #existing_value = 1 AND #the_bit_field = 0
BEGIN
RAISERROR('Fail.', 10, 1)
RETURN -1
END
-- Update the table as normal.
END
GO
Using a Trigger is kind of like trying to get the thief (the error) out of the house after he got in through the open front door (the SP's query). Lock the front door instead ;)

Answer 3

One way would be an instead of trigger that lets all the updates through except anything that changes the bitfield once it's equal to 1. In that case it lets all the updates through except the bitfield change.
CREATE TRIGGER OneWayBitChange
ON YourTable
INSTEAD OF UPDATE
AS
BEGIN
UPDATE YourTable SET /* update all fields from original update except bitfield */
Field1 = i.Field1,
Field2 = i.Field2,
Field3 = i.Field3
FROM YourTable
INNER JOIN Inserted i ON i.PrimaryKey = YourTable.PrimaryKey
UPDATE YourTable SET /* update bitfield only if it's not already a 1 */
BitField = i.BitField
FROM YourTable
INNER JOIN Inserted i ON i.PrimaryKey = YourTable.PrimaryKey
WHERE IsNull(YourTable.BitField,0) < 1
END
GO
The above will allow all updates accept the one field when going from 1 to anything else (0, or null).
If you want to cancel any update on that row when an attempt is made on the bit field you can modify the body like this:
UPDATE YourTable SET /* update all except bitfield changes from 1 to 0 */
Field1 = i.Field1,
Field2 = i.Field2,
Field3 = i.Field3,
BitField = i.BitField
FROM YourTable
INNER JOIN Inserted i ON i.PrimaryKey = YourTable.PrimaryKey
WHERE IsNull(BitField,0) = 0 OR IsNull(i.BitField,0) = 1