SQL Trigger Inconsistently firing - sql-server

Question

I have a SQL Trigger on a table that works... most of the time. And I cannot figure out why sometimes the fields are NULL
The trigger works by Updateing the LastUpdateTime whenever something is modified in the field, and the InsertDatetime when first Created.
For some reason this only seems to work some times.
ALTER TRIGGER [dbo].[DateTriggerTheatreListHeaders]
ON [dbo].[TheatreListHeaders]
AFTER INSERT,UPDATE
AS
BEGIN
SET NOCOUNT ON;
IF NOT EXISTS(SELECT * FROM DELETED)
BEGIN
UPDATE ES
SET InsertDatetime = Getdate()
,LastUpdateDateTime = Getdate()
FROM TheatreListHeaders es
JOIN Inserted I ON es.UNIQUETHEATRELISTNUMBER = I.UNIQUETHEATRELISTNUMBER
END
IF UPDATE(LastUpdateDateTime) OR UPDATE(InsertDatetime)
RETURN;
IF EXISTS (
SELECT
*
FROM
INSERTED I
JOIN
DELETED D
-- make sure to compare inserted with (same) deleted person
ON D.UNIQUETHEATRELISTNUMBER = I.UNIQUETHEATRELISTNUMBER
)
BEGIN
UPDATE ES
SET InsertDatetime = ISNULL(es.Insertdatetime,Getdate())
,LastUpdateDateTime = Getdate()
FROM TheatreListHeaders es
JOIN Inserted I ON es.UNIQUETHEATRELISTNUMBER = I.UNIQUETHEATRELISTNUMBER
END
END

Answer 1

A much simpler and efficient approach to do what you are trying to do, would be something like...
ALTER TRIGGER [dbo].[DateTriggerTheatreListHeaders]
ON [dbo].[TheatreListHeaders]
AFTER INSERT,UPDATE
AS
BEGIN
SET NOCOUNT ON;
--Determine if this is an INSERT OR UPDATE Action .
DECLARE #Action as char(1);
SET #Action = (CASE WHEN EXISTS(SELECT * FROM INSERTED)
AND EXISTS(SELECT * FROM DELETED)
THEN 'U' -- Set Action to Updated.
WHEN EXISTS(SELECT * FROM INSERTED)
THEN 'I' -- Set Action to Insert.
END);
UPDATE ES
SET InsertDatetime = CASE WHEN #Action = 'U'
THEN ISNULL(es.Insertdatetime,Getdate())
ELSE Getdate()
END
,LastUpdateDateTime = Getdate()
FROM TheatreListHeaders es
JOIN Inserted I ON es.UNIQUETHEATRELISTNUMBER = I.UNIQUETHEATRELISTNUMBER;
END

Answer 2

"If update()" is poorly defined/implemented in sql server IMO. It does not do what is implied. The function only determines if the column was set by a value in the triggering statement. For an insert, every column is implicitly (if not explicitly) assigned a value. Therefore it is not useful in an insert trigger and difficult to use in a single trigger that supports both inserts and updates. Sometimes it is better to write separate triggers.
Are you aware of recursive triggers? An insert statement will execute your trigger which updates the same table. This causes the trigger to execute again, etc. Is the (database) recursive trigger option off (which is typical) or adjust your logic to support that?
What are your expectations for the insert/update/merge statements against this table? This goes back to your requirements. Is the trigger to ignore any attempt to set the datetime columns directly and set them within the trigger always?
And lastly, what exactly does "works sometimes" actually mean? Do you have a test case that reproduces your issue. If you don't, then you can't really "fix" the logic without a specific failure case. But the above comments should give you sufficient clues. To be honest, your logic seems to be overly complicated. I'll add that it also is logically flawed in the way that it set insertdatetime to getdate if the existing value is null during an update. IMO, it should reject any update that attempts to set the value to null because that is overwriting a fact that should never change. M.Ali has provided an example that is usable but includes the created timestamp problem. Below is an example that demonstrates a different path (assuming the recursive trigger option is off). It does not include the rejection logic - which you should consider. Notice the output of the merge execution carefully.
use tempdb;
set nocount on;
go
create table zork (id integer identity(1, 1) not null primary key,
descr varchar(20) not null default('zippy'),
created datetime null, modified datetime null);
go
create trigger zorktgr on zork for insert, update as
begin
declare #rc int = ##rowcount;
if #rc = 0 return;
set nocount on;
if update(created)
select 'created column updated', #rc as rc;
else
select 'created column NOT updated', #rc as rc;
if exists (select * from deleted) -- update :: do not rely on ##rowcount
update zork set modified = getdate()
where exists (select * from inserted as ins where ins.id = zork.id);
else
update zork set created = getdate(), modified = getdate()
where exists (select * from inserted as ins where ins.id = zork.id);
end;
go
insert zork default values;
select * from zork;
insert zork (descr) values ('bonk');
select * from zork;
update zork set created = null, descr = 'upd #1' where id = 1;
select * from zork;
update zork set descr = 'upd #2' where id = 1;
select * from zork;
waitfor delay '00:00:02';
merge zork as tgt
using (select 1 as id, 'zippity' as descr union all select 5, 'who me?') as src
on tgt.id = src.id
when matched then update set descr = src.descr
when not matched then insert (descr) values (src.descr)
;
select * from zork;
go
drop table zork;