SQL Server: check constraint if relationship exists, else insert - sql-server

Question

I am trying to create a constraint to validate if a relation exists
I have tried to create a procedure and then use it in check constraint. Apparently that does not seem to work.
These are my tables:
STOCKITEMS table:
StockItemId INT
StockItemName VARCHAR
ColorId INT
COLOR table:
ColorId INT
ColorName VARCHAR
This is my stored procedure:
CREATE PROCEDURE USP_ValidateColor
(#Color NVARCHAR(50))
AS
IF NOT EXISTS(SELECT ColorName FROM WareHouse.Colors WHERE ColorName = #Color)
BEGIN
DECLARE #Id INT
SET #Id = (SELECT TOP(1) ColorId + 1 FROM Warehouse.Colors
ORDER BY ColorId DESC)
INSERT INTO Warehouse.Colors
VALUES (#Id, #Color)
PRINT 'Does not exist';
END;
ELSE
PRINT 'Exists';
So if a user insert into the table stock items, I want a check that checks if the colorId already exists in the color table
If it does not, then insert that colorname into colors and. I was thinking about using a constraint check with my procedure, but can't fix the query.

Answer 1

Don't use an SP to check a constraint, use a foreign key:
CREATE TABLE Colour (ColourID int PRIMARY KEY, --This should really have a name
ColourName varchar(20));
CREATE TABLE StockItem (StockItemID int PRIMARY KEY, --This should really have a name too
StockItemName varchar(20),
ColourID int);
ALTER TABLE dbo.StockItem ADD CONSTRAINT Colour_FK FOREIGN KEY (ColourID) REFERENCES dbo.Colour(ColourID);
Then, if you try to insert something into the StockItem table, it'll fail unless the colour exists:
INSERT INTO dbo.Colour (ColourID,
ColourName)
VALUES (1,'Green'),(2,'Blue');
GO
INSERT INTO dbo.StockItem (StockItemID,
StockItemName,
ColourID)
VALUES(1,'Paint',1); --works
GO
INSERT INTO dbo.StockItem (StockItemID,
StockItemName,
ColourID)
VALUES (1,'Wood Panels',3); --fails
GO
--clean up
DROP TABLE dbo.StockItem;
DROP TABLE dbo.Colour;

Answer 2

For checking, use a UNIQUE check constraint. If you want to insert a color only if it doesn't exist, use INSERT .. FROM .. WHERE to check for existence and insert in the same query.
The only "trick" is that FROM needs a table. This can be fixed using a table value constructor to create tables out of the values to insert. If the stored procedure accepts a table-valued parameter, there's no problem.
This example uses a LEFT JOIN to insert non-matching values :
declare #colors table (Color nvarchar(10) UNIQUE)
insert into #colors VALUES ('green')
select * from #colors;
insert into #Colors (Color)
select new.Color
from (VALUES ('red'),
('green')) new(Color)
left outer join #Colors old on old.Color=new.Color
where old.Color is NULL
-- (1 row affected)
insert into #Colors (Color)
select new.Color
from (VALUES ('red'),
('green')) new(Color)
left outer join #Colors old on old.Color=new.Color
where old.Color is NULL
-- (0 rows affected)
select * from #colors;
-- green
-- red
The same using a subquery:
insert into #Colors (Color)
select new.Color
from
(VALUES ('red'),
('green')) new(Color)
where not exists (select 1
from #colors
where color=new.Color);
By using the UNIQUE constraint we ensure that duplicate entries can't be inserted