Verification of adding an identical record in SQL - sql-server

Question

I need support, I have a procedure and I need it to return an error when I try to add the same name in the ProductName column:
USE [Northwind]
GO
/****** Object: StoredProcedure [dbo].[AddNewProduct] Script Date: 14.09.2021 18:15:53 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER procedure [dbo].[AddNewProduct]
#ProductName nvarchar(40), #SupplierID int, #CategoryID int, #QuantityPerUnit nvarchar(20), #UnitPrice money, #UnitsInStock smallint, #UnitsOnOrder smallint, #ReorderLevel smallint, #Discontinued bit
as
If #ProductName = #ProductName
Begin
Declare #count int, #ProductID int
Insert into Products
(
ProductName ,
SupplierID ,
CategoryID ,
QuantityPerUnit,
UnitPrice,
UnitsInStock,
UnitsOnOrder,
ReorderLevel,
Discontinued)
values
(#ProductName,
#SupplierID ,
#CategoryID ,
#QuantityPerUnit,
#UnitPrice,
#UnitsInStock,
#UnitsOnOrder,
#ReorderLevel,
#Discontinued )
End
else
Begin
DECLARE #ErrorMessage NVARCHAR(4000);
SET #ErrorMessage = 'DDDD';
RAISERROR (#ErrorMessage, 16, 1);
end
I want a message to appear when I try to add the same record
Can you tell me what I am doing wrong or what needs to be added / changed?

Answer 1

As has been mentioned in the comments, this logic has no right being in the procedure, it should be part of the table's definition, as a UNIQUE CONSTRAINT or UNIQUE INDEX. I'm going to use a UNIQUE CONSTRAINT here, which would mean the DDL for the CONSTRAINT would look like this:
ALTER TABLE dbo.Products ADD CONSTRAINT UQ_ProductName UNIQUE (ProductName);
Then, you just need your procedure to look like this:
ALTER PROCEDURE [dbo].[AddNewProduct] #ProductName nvarchar(40),
#SupplierID int,
#CategoryID int,
#QuantityPerUnit nvarchar(20), --Why is this an nvarchar if it's a quantity?
#UnitPrice money,
#UnitsInStock smallint,
#UnitsOnOrder smallint,
#ReorderLevel smallint,
#Discontinued bit AS
BEGIN
INSERT INTO dbo.Products(ProductName,
SupplierID,
CategoryID,
QuantityPerUnit,
UnitPrice,
UnitsInStock,
UnitsOnOrder,
ReorderLevel,
Discontinued)
VALUES (#ProductName, #SupplierID, #CategoryID, #QuantityPerUnit, #UnitPrice, #UnitsInStock, #UnitsOnOrder, #ReorderLevel, #Discontinued);
END;
GO
If you then attempt to insert a duplicate value you'll get the following error:
Violation of UNIQUE KEY constraint 'UQ_ProductName'. Cannot insert duplicate key in object 'dbo.Products'. The duplicate key value is ({Duplicate Product Name}).