MSSQL Update: output value before update - sql

Question

There is a table with IDU (PK) and stat columns. If first bit of stat is 1 I need to set it to 0 and run some stored procedure in this case only, otherwise I do nothing.
Here is the simple query for this
DECLARE #s INT
-- get the current value of status before update
SET #s = (SELECT stat FROM myTable
WHERE IDU = 999999999)
-- check it first bit is 1
IF (#s & 1) = 1
BEGIN
-- first bit is 1, set it to 0
UPDATE myTable
SET status = Stat & ~1
WHERE IDU = 999999999
-- first bit is one, in this case we run our SP
EXEC SOME_STORED_PROCEDURE
END
But I'm not sure that this query is optimal. I heard about OUTPUT parameter for UPDATE query but I found how to get inserted value. Is there a way to get a value that was before insert?

Answer 1

Yes, OUTPUT clause allows you to get the previous value before the update. You need to look at deleted and inserted tables.
DELETED
Is a column prefix that specifies the value deleted by the
update or delete operation. Columns prefixed with DELETED reflect the
value before the UPDATE, DELETE, or MERGE statement is completed.
INSERTED
Is a column prefix that specifies the value added by the insert or
update operation. Columns prefixed with INSERTED reflect the value
after the UPDATE, INSERT, or MERGE statement is completed but before
triggers are executed.
-- Clear the first bit without checking what it was
DECLARE #Results TABLE (OldStat int, NewStat int);
UPDATE myTable
SET Stat = Stat & ~1
WHERE IDU = 999999999
OUTPUT
deleted.Stat AS OldStat
,inserted.Stat AS NewStat
INTO #Results
;
-- Copy data from #Results table into variables for comparison
-- Assumes that IDU is a primary key and #Results can have only one row
DECLARE #OldStat int;
DECLARE #NewStat int;
SELECT #OldStat = OldStat, #NewStat = NewStat
FROM #Results;
IF #OldStat <> #NewStat
BEGIN
EXEC SOME_STORED_PROCEDURE;
END;

Answer 2

Regardless of optimal, this query is not 100% safe. This is because between SET #s =... and UPDATE myTable there is no guarantee the value of stat has not been changed. If this code runs multiple times it is possible to screw up if two cases execute deadly close for the same IDU. The first thread will do ok but the second one will not, since the first would change the stat after the second read it and before update it. A select statement does not lock beyond its own execution time even on SERIALIZABLE isolation.
To be safe, you need to lock the record BEFORE read it, and to do that you need an update statement, even fake:
DECLARE #s INT
BEGIN TRANSACTION
UPDATE myTable SET stat = stat WHERE IDU = 999999999 --now you row lock your row, make sure no other thread can move along
-- get the current value of status before update
SET #s = (SELECT stat FROM myTable
WHERE IDU = 999999999)
-- check it first bit is 1
IF (#s & 1) = 1
BEGIN
-- first bit is 1, set it to 0
UPDATE myTable
SET status = Stat & ~1
WHERE IDU = 999999999
-- first bit is one, in this case we run our SP
-- COMMIT TRANSACTION here? depends on what SOME_STORED_PROCEDURE does
EXEC SOME_STORED_PROCEDURE
COMMIT TRANSACTION --i believe here you release the row lock
I am not sure what you mean by "Is there a way to get a value that was before insert" because you only update and the only data, stat, you had already read from the old record regardless if you update or not.

Answer 3

You could do this with an INSTEAD OF UPDATE Trigger.