Get ROWCOUNT from INSTEAD OF TRIGGER - sql

Question

A legacy app does an INSERT on a table with an instead of trigger and subsequently uses the rowcount for further processing.
We now need to opt out of certain INSERTs with the use of an INSTEAD OF INSERT trigger.
The problem is that ##ROWCOUNT still returns the number of attempted inserts.
For example, a fictitious trigger that will never complete an insert might be
ALTER TRIGGER [dbo].[trig_ACCOUNT_CREDITS_RunningTotalINSERT]
ON [dbo].[ACCOUNT_CREDITS]
INSTEAD OF INSERT
AS
BEGIN
--tried with NOCOUNT ON and OFF
SET NOCOUNT OFF;
--This is an example of the branching logic that might determine
--whether or not to do the INSERT
IF 1=2 --no insert will ever occur (example only)
BEGIN
INSERT INTO dbo.ACCOUNT_CREDITS (COL1, COL2)
SELECT COL1, COL2 from INSERTED
END
END
and some INSERT statements might be
--No rows will be inserted because value of COL1 < 5
INSERT INTO dbo.ACCOUNT_CREDITS (COL1, COL2) VALUES ( 3, 3)
--We would assume row count to be 0, but returns 1
select ##ROWCOUNT
--No rows will be inserted because value of COL1 < 5
INSERT INTO dbo.ACCOUNT_CREDITS (COL1, COL2)
SELECT 1, 1
union all
SELECT 2, 2
--We would assume row count to be 0, but returns 2
select ##ROWCOUNT
I can work around the issue, but it bothers me that I can't trust ##ROWCOUNT. I can find no reference to this issue on SO or those other knowledge banks. Is this simply a case of TRIGGERS ARE EVIL?
Can I affect ##ROWCOUNT?

Answer 1

Some statements may change ##ROWCOUNT inside the trigger.
Statement
SELECT * FROM INSERTED WHERE COL1 < 5
executes and set ##ROWCOUNT to 1
Put statement
SET NOCOUNT ON;
then
IF NOT EXISTS (SELECT * FROM INSERTED WHERE COL1 < 5)
BEGIN
SET NOCOUNT OFF;
INSERT INTO dbo.ACCOUNT_CREDITS (COL1, COL2)
SELECT COL1, COL2 from INSERTED
END

Answer 2

The Problem
I need information in the context of the main process that is only available in the context of the trigger.
The Solution
Whether getting ##ROWCOUNT or anything else from a trigger, or even passing information to a trigger, there are two methods that allow for sharing information with triggers:
SET CONTEXT_INFO / CONTEXT_INFO()
Local Temporary Tables (i.e. tables with names starting with a single #: #tmp)
I posted an example of using CONTEXT_INFO in an answer on a related question over at DBA.StackExchange: Passing info on who deleted record onto a Delete trigger. There was a discussion in the comments on that answer related to possible complications surrounding CONTEXT_INFO, so I posted another answer on that question using a temporary table instead.
Since that example dealt with sending information to a trigger, below is an example of getting information from a trigger as that is what this question is about:
First: Create a simple table
CREATE TABLE dbo.InsteadOfTriggerTest (Col1 INT);
Second: Create the trigger
CREATE TRIGGER dbo.tr_InsteadOfTriggerTest
ON dbo.InsteadOfTriggerTest
INSTEAD OF INSERT
AS
BEGIN
PRINT 'Trigger (starting): ' + CONVERT(NVARCHAR(50), ##ROWCOUNT);
SET NOCOUNT ON; -- do AFTER the PRINT else ##ROWCOUNT will be 0
DECLARE #Rows INT;
INSERT INTO dbo.InsteadOfTriggerTest (Col1)
SELECT TOP (5) ins.Col1
FROM inserted ins;
SET #Rows = ##ROWCOUNT;
PRINT 'Trigger (after INSERT): ' + CONVERT(NVARCHAR(50), #Rows);
-- make sure temp table exists; no error if table is missing
IF (OBJECT_ID('tempdb..#TriggerRows') IS NOT NULL)
BEGIN
INSERT INTO #TriggerRows (RowsAffected)
VALUES (#Rows);
END;
END;
Third: Do the test
SET NOCOUNT ON;
IF (OBJECT_ID('tempdb..#TriggerRows') IS NOT NULL)
BEGIN
DROP TABLE #TriggerRows;
END;
CREATE TABLE #TriggerRows (RowsAffected INT);
INSERT INTO dbo.InsteadOfTriggerTest (Col1)
SELECT so.[object_id]
FROM [master].[sys].[objects] so;
PRINT 'Final ##ROWCOUNT (what we do NOT want): ' + CONVERT(NVARCHAR(50), ##ROWCOUNT);
SELECT * FROM #TriggerRows;
Output (in Messages tab):
Trigger (starting): 91
Trigger (after INSERT): 5
Final ##ROWCOUNT (what we do NOT want): 91
Results:
RowsAffected
5