DELETE WHERE NOT EXISTS Not Being Applied Correctly - tsql

Question

I'm using NOT EXISTS during a DELETE statement in a stored procedure and the not exists is not being applied to the data.
Using the following example data:
CREATE TABLE Region
(
RegionID INT IDENTITY(1,1)
,RegionName VARCHAR(25)
)
GO
INSERT INTO Region(RegionName)
VALUES ('East Coast')
,('Mid West')
,('West Coast')
GO
CREATE TABLE Customer
(
CustomerID INT IDENTITY(1,1)
,FirstName VARCHAR(5)
,Region INT
)
GO
INSERT INTO Customer(FirstName,Region)
VALUES('Tom',1)
,('Mike',2)
,('Jean',3)
GO
CREATE TABLE Orders
(
OrderID INT IDENTITY(1,1)
,CustomerID INT
,OrderAmount INT
,OrderDate DATE
)
GO
INSERT INTO Orders(CustomerID,OrderAmount,OrderDate)
VALUES(1,10,'2018-11-30')
,(2,12,'2018-11-30')
,(2,15,'2018-12-01')
,(2,8,'2018-12-02')
,(2,11,'2018-12-03')
,(3,13,'2018-12-01')
,(3,20,'2018-12-03')
GO
Using that data I'm trying to create a procedure that does the following:
CREATE PROCEDURE udsp_GetOrdersOfXAmount #OrderAmount INT, #RegionID INT = 0
AS
BEGIN
DECLARE #ProcedureTemp TABLE
(
OrderID INT
,CustomerID INT
,OrderAmount INT
,OrderDate DATE
)
INSERT INTO #ProcedureTemp(OrderID,CustomerID,OrderAmount,OrderDate)
SELECT * FROM Orders WHERE OrderAmount >= #OrderAmount
--Do several other UPDATES/ DELETES to #ProcedureTemp
--This is where the issue lies
IF #RegionID > 0
BEGIN
DELETE T FROM #ProcedureTemp T
WHERE NOT EXISTS
(
SELECT *
FROM Customer C
JOIN #ProcedureTemp T ON T.CustomerID = C.CustomerID
WHERE C.Region = #RegionID
)
END
SELECT * FROM #ProcedureTemp
END
GO
If you execute the procedure with the #RegionID parameter populated, you will see the procedure is not honoring the filter by region.
E.G.
EXEC udsp_GetOrdersOfXAmount 10,3
However, if you run the sub query used in the DELETE statement as its own query, you will see the WHERE clause logic provided is working. I don't understand why the it isn't working when used with NOT EXISTS in the DELETE statement.
DECLARE #OrderAmount INT = 10, #RegionID INT = 3
DECLARE #ProcedureTemp TABLE
(
OrderID INT
,CustomerID INT
,OrderAmount INT
,OrderDate DATE
)
INSERT INTO #ProcedureTemp(OrderID,CustomerID,OrderAmount,OrderDate)
SELECT * FROM Orders WHERE OrderAmount >= #OrderAmount
SELECT *
FROM Customer C
JOIN #ProcedureTemp T ON T.CustomerID = C.CustomerID
WHERE C.Region = #RegionID
Thank you in advance for any help you can provide.

Answer 1

You don't need the join in the inner query.
The fact that you are using the same alias for the outer query and the inner one is confusing to me, I'm guessing SQL Server also should have a problem with it.
Try writing it like this:
DELETE T
FROM #ProcedureTemp T
WHERE NOT EXISTS
(
SELECT *
FROM Customer C
-- You already have the T from the outer statement
WHERE T.CustomerID = C.CustomerID
AND C.Region = #RegionID
)