Incorrect syntax near case in trigger - sql-server

Question

I have a Reservations table with the following columns
Reservation_ID
Res_TotalAmount - money
Res_StartDate - datetime
IsDeleted - bit column with default value - false
So when a user tries to delete his reservation I've created a trigger that instead of delete - he just updates the value of column IsDelete to true;
So far so good - but this tourist may owe some compensation to the firm, for example when he has cancelled his reservation 30 days to 20 days from the start_date of the reservation - he owes 30% of the Res_TotalAmount and so on
And here is my trigger
Create Trigger tr_TotalAMountUpdateAfterInsert on RESERVATIONS after Delete
As
Begin
Declare #period int
Declare #oldResAmount int
Declare #newAmount money
Declare #resID int
Select #resID = Reservation_ID from deleted
select #oldResAmount = Res_TotalAmount from deleted
Select #period= datediff (day,Res_StartDate,GETDATE()) from deleted
case
#period is between 30 and 20 then #newAmount=30%*#oldResAmount
#period is between 20 and 10 then #newAmount=50%*#oldResAmount
end
exec sp_NewReservationTotalAmount #newAmount #resID
End
GO
As I have to use both triggers and stored procedure you see that I call at the end of the trigger one stored procedure which just updates Res_TotalAmount column
Create proc sp_NewReservationTotalAmount(#newTotalAmount money, #resID)
As
declare #resID int
Update RESERVATIONS set Res_TotalAmount=#newTotalAmount where Reservation_ID=resID
GO
So my first problem is that it gives me incorrect syntax near case
And my second - I would appreciate suggestions how to make both the trigger and stored procedure better.

Answer 1

Your fundamental flaw is that you seem to expect the trigger to be fired once per row - this is NOT the case in SQL Server. Instead, the trigger fires once per statement, and the pseudo table Deleted might contain multiple rows.
Given that that table might contain multiple rows - which one do you expect will be selected here??
Select #resID = Reservation_ID from deleted
select #oldResAmount = Res_TotalAmount from deleted
Select #period= datediff (day,Res_StartDate,GETDATE()) from deleted
It's undefined - you might get the values from arbitrary rows in Deleted.
You need to rewrite your entire trigger with the knowledge the Deleted WILL contain multiple rows! You need to work with set-based operations - don't expect just a single row in Deleted !
Also: the CASE statement in T-SQL is just intended to return an atomic value - it's not a flow control statement like in other languages, and it cannot be used to execute code. So your CASE statement in your trigger is totally "lost" - it needs to be used in an assignment or something like that ....

Answer 2

1) Here is the correct syntax for the CASE statement. Note that:
I changed the order of your comparisons with the CASE statement; the smaller value has to come first.
I have included an "ELSE" case so you don't wind up with an undefined value when #period is not within your given ranges
SELECT #newAmount =
CASE
WHEN #period between 10 and 20 then 0.5 * #oldResAmount
WHEN #period between 20 and 30 THEN 0.3 * #oldResAmount
ELSE #oldResAmount
END
2) You are going to have an issue with this trigger if ever a delete statement affects more than one row. Your statements like "SELECT #resID = Reservation_ID from deleted;" will simply assign one value from the deleted table at random.
EDIT
Here is an example of a set-based approach to your problem that will still work when multiple rows are "deleted" within the transaction (example code only; not tested):
Create Trigger tr_TotalAMountUpdateAfterInsert on RESERVATIONS after Delete
As
Begin
UPDATE RESERVATIONS
SET Res_TotalAmount =
d.Res_TotalAmount * dbo.ufn_GetCancellationFactor(d.Res_StartDate)
FROM RESERVATIONS r
INNER JOIN deleted d ON r.Reservation_ID = d.Reservation_ID
End
GO
CREATE FUNCTION dbo.ufn_GetCancellationFactor (#scheduledDate DATETIME)
RETURNS FLOAT AS
BEGIN
DECLARE #cancellationFactor FLOAT;
DECLARE #period INT = DATEDIFF (DAY, #scheduledDate, GETDATE());
SELECT #cancellationFactor =
CASE
WHEN #period <= 10 THEN 1.0 -- they owe the full amount (100%)
WHEN #period BETWEEN 11 AND 20 THEN 0.5 -- they owe 50%
WHEN #period BETWEEN 21 AND 30 THEN 0.3 -- they owe 30%
ELSE 0 -- they owe nothing
END
RETURN #cancellationFactor;
END;
GO

Answer 3

About the case:
The syntax is wrong. Even if it worked (see below) you'd be missing the WHENs:
case
WHEN #period is between 30 and 20 then #newAmount=30%*#oldResAmount
WHEN #period is between 20 and 10 then #newAmount=50%*#oldResAmount
end
Yet, the case statement can not be used this way. In the context where you want it, you need to use if. It's not like the switch statement in C++/C# for example. You can only use it in queries like
SELECT
case
WHEN #period is between 30 and 20 then value1
WHEN #period is between 20 and 10 then value2
end
Having said the above: I didn't actually read all your code. But now that I've read some of it, it is really important to understand how triggers work in SQL Server, as mark_s says.