Multiple Rows with in SQL INSTEAD of Trigger - sql

Question

looking a bit of direction or guidance as to why I’m getting multiple rows using my trigger. Basically I have a web app that controls Asset Types (i.e Laptops, Phones etc), what I’m trying to do with this trigger is when the Asset Type Name (at_typedesc) changes that I log to an audit table (in this case sql_log) what the old name was and what the new name is.
This is working, but for some reason I get multiple lines written at the INSERT TO SQL_LOG statement. It does write the old name & new name, but then I’ll get 3 additional rows which has the old name showing the new name...
This is currently on a 2008 SQL Server.
-- create the trigger
go
create trigger trg_InsteadOfUpdate on [dbo].[lkp_asset_types]
instead of update
as
begin
DECLARE #triggerAction varchar(1)
-- determine the TRIGGER action
-- this allows us to tell if its an INSERT or an UPDATE
SELECT #triggerAction = CASE
WHEN EXISTS(SELECT 1 FROM INSERTED)
AND EXISTS(SELECT 1 FROM deleted) THEN 'U'
WHEN EXISTS(SELECT 1 FROM inserted) THEN 'I'
ELSE 'D' END;
-- get the orginally asset name from the DELETED table
-- this contains the rows as they were BEFORE the UPDATE Statement
DECLARE #orgAssetTypeName varchar(255)
SET #orgAssetTypeName = (SELECT top 1 at_typedesc from lkp_asset_types WHERE at_id = (select at_id from deleted))
-- UPDATE to the new asset name based on the NEW value in the INSERTED Table
update lkp_asset_types
set at_typedesc = (select at_typedesc from inserted)
where at_id = (select at_id from inserted)
-- get the new asset name from the INSERTED table
-- this contains the rows as they were AFTER the UPDATE Statement
DECLARE #newAssetTypeName varchar(255)
SET #newAssetTypeName = (SELECT top 1 at_typedesc from lkp_asset_types WHERE at_id = (select at_id from inserted))
insert into sql_log
(sql_log)
values ('SQL PRE Changed from : ' + #orgAssetTypeName + ' to: ' + #newAssetTypeName + '. Action = ' + #triggerAction)
end
go

Answer 1

Logic like this in a trigger in SQL Server is just broken:
where at_id = (select at_id from inserted)
I really wish the SQL Server parser issued a warning when encountering such constructs.
There is no guarantee that inserted has only one value (nor deleted).
That is how SQL Server defines triggers: as set operations. If multiple rows are inserted at the same, then the inserted and deleted "tables" have multiple rows.
That part is simple. You will need to rewrite the trigger to take this into account.

Answer 2

On checking my web code the update button was performing additional updates, this caused the trigger to fire more than once thus causing duplicate rows.

Answer 3

create table dbo.lkp_asset_types_test
(
at_id int identity,
at_typedesc varchar(100)
)
go
create trigger trg_InsteadOfUpdate_test on [dbo].[lkp_asset_types_test]
instead of update
as
begin
select 'trigger fired!!!!'
if not exists(select * from inserted)
and not exists(select * from deleted)
begin
return;
end
--update (maybe only the diffs?)
update t
set at_typedesc = i.at_typedesc
from dbo.lkp_asset_types_test as t
join inserted as i on t.at_id = i.at_id;
--where t.at_typedesc <> i.at_typedesc & nulls??
--insert into sql_log(sql_log)
select
'SQL PRE Changed from : ' + isnull(d.at_typedesc, '*null*') + ' to: ' + isnull(i.at_typedesc, '*null*') + '. Action = U'
from inserted as i
join deleted as d on i.at_id = d.at_id
--where i.at_typedesc <> d.at_typedesc & nulls ??
end
go
insert into dbo.lkp_asset_types_test(at_typedesc) values ('A'), ('B'), ('C'), ('D'), (NULL);
go
update dbo.lkp_asset_types_test
set at_typedesc = case when at_id%2=0 then isnull(at_typedesc, 'X') else isnull(at_typedesc, '') + 'xyz' end
go
select *
from dbo.lkp_asset_types_test;
go
update dbo.lkp_asset_types_test
set at_typedesc = case when at_id%2=0 then at_typedesc else at_typedesc + 'xyz' end
where 1=2
go
--
drop table lkp_asset_types_test;