Whats Wrong with the Stored Procedure below? - sql-server

Question

alter procedure product_sales
#productid int,
#quantitysell int
as
begin
declare #productname varchar(20)
declare #quantityavailable int
select #quantityavailable=quantityav from products_trycatch
select #productname=productname from products_trycatch where productid=#productid
select #productid=productid from products_trycatch
if(not exists(select #productid from products_trycatch)
begin
raiserror('Product does not exist',16,1)
end
else
begin
if (#quantitysell>#quantityavailable)
begin
print 'Stock not available'
end
else
begin
-------------
update products_trycatch set quantityav=quantityav-#quantitysell
insert into product_log values(#productid,#productname,#quantitysell)
end
Please let me know where the mistake is. What i am trying to do is. one table contains stock available for a product, when i execute the SP i mentioned which product and how many quantity is being sold, The sp deducts that value from the available quantity table and updates in a new table the quantity sold.

Answer 1

You're thinking very procedurally, laying things out step by step, but missing parts out from several of your steps.
Don't write several queries to retrieve individual pieces of data. Think about the problem as a whole1:
alter procedure product_sales
#productid int,
#quantitysell int
as
begin
declare #rc int
update products_trycatch
set quantityav = quantityav - #quantitysell
where productid = #productid and quantityav > #quantitysell
set #rc = ##ROWCOUNT
if #rc = 1
begin
insert into product_log
select #productid,productname,#quantitysell
from product_trycatch
where productid = #productid
end
else
begin
if not exists(select * from products_trycatch where productid = #productid)
begin
raiserror('Product does not exist',16,1)
end
else
begin
print 'Stock not available'
end
end
Not how all of my queries target the row in product_trycatch with the matching productid column - several of yours don't which means that they're going to be assigning non-deterministic values to your variables, or updating all rows in product_trycatch and deducting the #quantitysell value.
1For bonus points we could write a single update with case and an output clause that would eliminate the need to perform any re-querying of the product_trycatch table for the product name or in the failure path, but it's probably not worth the additional complexity here. This query shows the general technique:
declare #products table (ID int not null, Quantity int not null, Name varchar(20) not null)
insert into #products (ID,Quantity,Name) values (1,15,'Fred')
declare #ID int
declare #Qty int
declare #RefData table (OldQuantity int not null, NewQuantity int not null,
Name varchar(20) not null)
select #ID = 1, #Qty = 20
update #products
set Quantity = CASE WHEN Quantity >= #Qty THEN Quantity - #Qty ELSE Quantity END
output deleted.Quantity,inserted.Quantity, inserted.Name into #RefData
where ID = #ID
select * from #RefData
where you can play with the #ID and #Qty values to see the various outcomes reflected in #RefData when the quantity requested is higher than that which is available or where the product does or doesn't exist.

Answer 2

Formatted your code and added missing words -
alter procedure product_sales
#productid int,
#quantitysell int
as
begin
declare #productname varchar(20)
declare #quantityavailable int
select #quantityavailable=quantityav from products_trycatch
select #productname=productname from products_trycatch where productid=#productid
select #productid=productid from products_trycatch
if not exists(select #productid from products_trycatch)
begin
raiserror('Product does not exist',16,1)
end
else if (#quantitysell>#quantityavailable)
begin
print 'Stock not available'
end
else
begin
-------------
update products_trycatch set quantityav=quantityav-#quantitysell
insert into product_log values(#productid,#productname,#quantitysell)
end
end

Answer 3

Here is the Formatted code
alter procedure product_sales
#productid int,
#quantitysell int
as
begin
declare #productname varchar(20)
declare #quantityavailable int
select #quantityavailable=quantityav from products_trycatch
select #productname=productname from products_trycatch where productid=#productid
select #productid=productid from products_trycatch
if(not exists(select #productid from products_trycatch))
begin
raiserror('Product does not exist',16,1)
end
else if (#quantitysell>#quantityavailable)
begin
print 'Stock not available'
end
else
begin
-------------
update products_trycatch set quantityav=quantityav-#quantitysell
insert into product_log values(#productid,#productname,#quantitysell)
end
end

Answer 4

Just Modified your Stored Proc have a look at it
ALTER PROCEDURE Product_sales
(#Productid INT,
#Quantitysell INT
)
AS
BEGIN TRY
DECLARE #Productname VARCHAR(20)
DECLARE #Quantityavailable INT
SELECT #Quantityavailable = Quantityav
FROM Products_trycatch
SELECT #Productname = Productname
FROM Products_trycatch
WHERE Productid = #Productid
SELECT #Productid = Productid
FROM Products_trycatch
IF( #Productid IS NULL )
BEGIN
RAISERROR('Product does not exist',16,1)
END
ELSE IF ( #Quantitysell > #Quantityavailable )
BEGIN
PRINT 'Stock not available'
END
ELSE
BEGIN
DECLARE #Is_transcount INT
BEGIN TRANSACTION
SET #Is_transcount=1
UPDATE Products_trycatch
SET Quantityav = Quantityav - #Quantitysell;
INSERT INTO Product_log
VALUES (#Productid,
#Productname,
#Quantitysell)
COMMIT TRANSACTION
SET #Is_transcount=0
END
END TRY
BEGIN CATCH
IF( ##Trancount > 0 )
ROLLBACK TRANSACTION
DECLARE #Error_message VARCHAR(Max),
#Error_number INT,
#Error_procedure VARCHAR(100)
SELECT #Error_message = ERROR_MESSAGE(),
#Error_number = ERROR_NUMBER(),
#Error_procedure = ERROR_PROCEDURE()
END CATCH

Answer 5

The SP is open to a Parameter Sniffing issue.
You re-assign the value of #productid part way through your code.
From this point forward SQL will be making more assumptions about what plan to use because it no longer knows the actual value.