Sometimes SQL Server trigger is not firing - sql

Question

I created a trigger on my recharge table. It updates the balance of onaccountregistry table.
But sometimes when inserting rows into my recharge table it is not firing the trigger. Then values are mismatch. This recharge table insert rows every time.
I created the trigger as follows. This is not a replicated table. I'm using SQL Server 2008 Enterprise edition.
Please help me solve this matter
CREATE TRIGGER [dbo].[RechargeRefund]
ON [dbo].[ISRecharge]
FOR INSERT
AS
declare #tin char(9)
declare #depocd char(1)
declare #oldvalue money
declare #newvalue money
begin
select #tin = inserted.tin_subtin from inserted
select #depocd = inserted.updatetype from inserted
select #newvalue = inserted.DepositAmt from inserted
select #oldvalue = Totdeposit from ISOnAcctRegistry where tin_subtin = #tin
end
if #depocd ='1'
begin
update ISOnAcctRegistry
set Totdeposit = #oldvalue + #newvalue
where tin_subtin = #tin
end
if #depocd ='2'
begin
update ISOnAcctRegistry
set Totdeposit = #oldvalue - #newvalue
where tin_subtin = #tin
end
GO

Answer 1

As #marc points out, writing assuming a single row in inserted is bad - it can even be possible for your 3 selects from inserted to assign values to your 3 variables from 3 different (arbitrary) rows from inserted.
What you probably want is:
update i1
set Totdeposit = Totdesposit + t2.Total
from ISOnAcctRegistry i1
inner join
(select
tin_subtin,
SUM(CASE updatetype
WHEN 1 THEN DepositAmt
WHEN 2 THEN -DepositAmt
ELSE 0 END) as Total
from inserted
group by tin_subtin) t2
on
i1.tin_subtin = t2.tin_subtin
But you might be able to replace this work (and this column in ISOnAcctRegistry) with an indexed view built on ISRecharge - with some limitations, you can build a view that automatically performs a SUM across the rows in ISRecharge, and SQL Server would take responsibility for maintaining the value in the background for you.
Obviously, at present, your trigger doesn't account for any UPDATE or DELETE activity on ISRecharge. An indexed view would.

Answer 2

Well, of course it won't work - you're assuming that the trigger fires once per row inserted.
But that's NOT the case.
The trigger fires once per INSERT batch, and the pseudo-table Inserted might contain multiple rows!
If you did get an INSERT with more than one row - which row did your statements here select ?
select #tin = inserted.tin_subtin from inserted
select #depocd = inserted.updatetype from inserted
select #newvalue = inserted.DepositAmt from inserted
select #oldvalue = Totdeposit from ISOnAcctRegistry where tin_subtin = #tin
You need to rewrite your trigger so that it will handle multiple rows in Inserted - then it'll work every time.