How to create a User Defined Function for SQL server - sql

Question

I have a fairly large stored procedure that I'm trying to simplify for readability.
It contains many union clauses with statements like the one shown below:
Fig.1
SELECT COUNT(1) AS Total
FROM Orders
WHERE (NOT EXISTS (
SELECT 1
FROM (
SELECT Id
FROM OrderLineItems
WHERE Orders.Id = Order_Id) AS Sub
WHERE EXISTS (
SELECT 1
FROM NormalizedLineItems
WHERE (Sub.Id = OrderLineItem_Id)
AND (OutOfStock = 1))))
AND (EXISTS (
SELECT 1 AS Total
FROM OrderShipments
WHERE (Orders.Id = Order_Id)
AND (CarrierApproved = 0)))
AND (IsQuote = 0)
AND (Cancelled = 0)
AND (Archived = 0)
AND (Completed = 0)
AND (Holding = 0)
However there are many reoccurring patterns in each statement.
The following pattern turns up several times Fig.2
Fig.2
WHERE (NOT EXISTS (
SELECT 1
FROM (
SELECT Id
FROM OrderLineItems
WHERE Orders.Id = Order_Id) AS Sub
WHERE EXISTS (
SELECT 1
FROM NormalizedLineItems
WHERE (Sub.Id = OrderLineItem_Id)
AND (OutOfStock = 1))))
I'm trying to (for readability purposes) reduce the code in the master stored procedure
So i thought id performance test a UDF, I've come up with the following Fig.3
Fig.3
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE FUNCTION TestFunction (#OrderId int)
RETURNS TABLE
AS
RETURN
(
-- Add the SELECT statement with parameter references here
SELECT 1 AS Total
FROM (
SELECT OrderLineItems.Id AS Id
FROM OrderLineItems
WHERE #OrderId = Order_Id) AS Sub
WHERE EXISTS (
SELECT 1 AS Total
FROM NormalizedLineItems
WHERE (Sub.Id = OrderLineItem_Id)
AND (OutOfStock = 1)))
GO
All though the above compiles, I'm not really sure I'm on the right track, I'm having all sorts of problems trying to apply the above UDF to the original query.
I am seeking a concrete example of how to abstract Fig.2 from Fig.1 into a UDF so I can at least performance-test the solution to see if it's worthwhile.
Note: I do know user defined functions can be a performance nightmare, however I'm not even at a stage where I can test.

Answer 1

Create an order_ids table with only one column named order_id
Insert into order_ids
select order_id from
FROM Orders
WHERE (NOT EXISTS (
SELECT 1
FROM (
SELECT Id
FROM OrderLineItems
WHERE Orders.Id = Order_Id) AS Sub
WHERE EXISTS (
SELECT 1
FROM NormalizedLineItems
WHERE (Sub.Id = OrderLineItem_Id)
AND (OutOfStock = 1))))
Then you can simplify your Sql like this:
SELECT COUNT(1) AS Total
FROM Orders
join order_ids
on order_ids.order_id = Orders.order_id
...
If your reoccurring statement only in one query,
Common Table Expression is the best choice:
with CTE_order_ids as
(select order_id from
FROM Orders
WHERE (NOT EXISTS (
SELECT 1
FROM (
SELECT Id
FROM OrderLineItems
WHERE Orders.Id = Order_Id) AS Sub
WHERE EXISTS (
SELECT 1
FROM NormalizedLineItems
WHERE (Sub.Id = OrderLineItem_Id)
AND (OutOfStock = 1))))
)
SELECT COUNT(1) AS Total
FROM Orders
join CTE_order_ids
on order_ids.order_id = Orders.order_id
...