IF statement with SQL Server trigger - sql

Question

I am a beginner with SQL and I'm trying to create a trigger to fire with an IF statement. Below is the trigger:
CREATE TRIGGER [Vector].[trg_insert_id]
ON [vector].[a69]
FOR INSERT
AS
BEGIN
SET NOCOUNT ON;
DECLARE #OID int
SELECT #OID = OBJECTID FROM INSERTED
DECLARE #siteID float
SELECT #siteID = MAX([SiteID]) FROM [vector].[a69]
SELECT #siteID
IF #OID NOT IN(select OBJECTID from vector.a69)
BEGIN
UPDATE [vector].[a69]
SET SiteID = #siteID + 0.00001
WHERE OBJECTID IN (#OID)
END
END
What I'm trying to have the trigger do is, after an insert to the table, search to see if the inserted OBJECTID already exists in the table. If it does not, update SiteID field by 0.00001.
When I have the IF statement, the trigger does not work. If I remove the IF statement, the trigger works, but it increments the SiteID field for other records with the same OBJECTID.
Any suggestions on what I'm doing wrong?

Answer 1

NOT IN can be dangerous to use. If the subquery returns even a single NULL value, then the results is never true. This may be the cause of your problem. One simple fix is:
IF #OID NOT IN (select OBJECTID from vector.a69 where OBJECTID is not null)
. . .
I think a better approach is to use NOT EXISTS:
IF NOT EXISTS (select 1 from vector.a69 where OBJECTID = #OID)

Answer 2

Might as well learn to use triggers correctly. First, if you ever find yourself writing something like:
DECLARE #OID int
SELECT #OID = OBJECTID FROM INSERTED
then your trigger is broken . It must be able to handle multiple values in the inserted or deleted pseudo-tables:
Next this part doesn't make logical sense
IF #OID NOT IN(select OBJECTID from vector.a69)
BEGIN
UPDATE [vector].[a69]
SET SiteID = #siteID + 0.00001
WHERE OBJECTID IN (#OID)
END
IF #OID doesn't exist in table vector.a60 then you can't update it. I am partly guessing what you want to do but I think this is what you actually want in the trigger.
CREATE TRIGGER [Vector].[trg_insert_id]
ON [vector].[a69]
FOR INSERT
AS
BEGIN
SET NOCOUNT ON;
UPDATE [vector].[a69]
SET SiteID = #siteID + 0.00001
WHERE OBJECTID IN (SLECT OID FROM inserted)
END