MS SQL Constraint on a column - sql-server

Question

I have a table called Products this is how it looks like and I am trying to create a constraint on [IsDefaultProductKey] column, that any time a value is added to it, it needs be an active product key.
CREATE TABLE [dbo].[Products](
[ProductId] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](64) NOT NULL,
[IsActive] [bit] NOT NULL,
[IsDefaultProductKey] [int] NULL,
CONSTRAINT [PK_dbo.Products] PRIMARY KEY CLUSTERED
(
[ProductId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[Products] ADD CONSTRAINT [DF_products_IsActive] DEFAULT ((1)) FOR [IsActive]
GO
ALTER TABLE [dbo].[Products] WITH CHECK ADD CONSTRAINT [FK_Products_Product_IsDefaultProductKey] FOREIGN KEY([IsDefaultProductKey])
REFERENCES [dbo].[Products] ([ProductId])
GO
ALTER TABLE [dbo].[Products] CHECK CONSTRAINT [FK_Products_Product_IsDefaultProductKey]
GO
If these are the entries in the table, row 4 should not be allowed to have a value of 1, since 1 is inactive. How can I go about adding a constraint on the table for that
ProductId Name IsActive IsDefaultProductKey
1 Test1 0 NULL
2 Test2 1 NULL
3 Test3 0 2
4 Test4 0 1 (Should not let me do this)
Based on suggestion, I created this UDF. But still not acting 100% the way I want it.. Please suggest.
CREATE TABLE [dbo].[Products]( [ProductId] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](64) NOT NULL,
[IsActive] [bit] NOT NULL,
[IsDefaultProductKey] [int] NULL,
)
go
Create FUNCTION dbo.CheckProduct (#IsDefaultProductKey int)
RETURNS int
AS
BEGIN
DECLARE #retval int
SELECT #retval = 0
Select #retval = 1
FROM [Products]
WHERE ProductId = #IsDefaultProductKey and IsActive = 1
RETURN #retval
END;
GO
--Select CheckProduct(1)
ALTER TABLE [Products]
ADD CONSTRAINT chkActiveProduct
CHECK (IsDefaultProductKey = null or dbo.CheckProduct(IsDefaultProductKey) = 1);
go

Answer 1

You can use a CHECK CONSTRAINT that calls a UDF that queries the table to see if the ProductId referenced by IsDefaultProductKey is Active or not.
EDIT:
Since you need the constraint to check both ways, you would create a UDF that has parameters for ProductId, IsActive and IsDefaultProductKey.
Inside the function, if there is a non-NULL value for IsDefaultProductKey, then you need to query the table to see if the row with that ProductId is Active. If not, then the function needs to return false.
ALSO, if the IsActive parameter is passed a value of 0, then you need to check the table to make sure that no row has a IsDefaultProductKey equal to the value of the ProductId parameter. If there is such a row, then the function needs to return false.
But if neither of those cases occur, the function returns true, and in the CHECK CONSTRAINT, you then just test to see if the function returns true.

Answer 2

I did not understand your questions completely. However looks like you want to apply a check constraint based on the value of other column. The issue which I see in your SQL is you are applying a column level constraint, while I think you need to apply a table level constraint. Please see below sample based on my understanding of your question.
CREATE TABLE [dbo].[Products](
[ProductId] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](64) NOT NULL,
[IsActive] [bit] NOT NULL,
[IsDefaultProductKey] [int] NULL,
CONSTRAINT ck_contraint CHECK
(
(IsActive = 1 AND (IsDefaultProductKey>0) )
)
)

Answer 3

I think you need a trigger not a constraint to do this. Something like:
CREATE OR ALTER TRIGGER DefaultNotActive ON [dbo].[Products]
AFTER INSERT, UPDATE
AS
IF EXISTS (SELECT *
FROM [dbo].[Products] p
JOIN inserted AS i
ON p.[ProductId] = i.[IsDefaultProductKey]
WHERE p.[IsActive] = 0
)
BEGIN
RAISERROR ('Default Product is inactive.', 16, 1);
ROLLBACK TRANSACTION;
RETURN
END;
It might need to be more complex if you also need the default product key to exist. Currently this will allow inserts\updates where the default product key is an id that does not have an existing row in the table

Answer 4

Thanks to Tab Allemnan, Here is the solution I found. Works both ways.
Create FUNCTION CheckProduct (#IsDefaultProductKey int, #ProductId int, #IsActive bit)
RETURNS bit
AS
BEGIN
BEGIN
DECLARE #ret bit;
if (#IsDefaultProductKey is not NULL)
begin
SELECT #ret = 1
FROM [Products] p
WHERE p.ProductID = #IsDefaultProductKey
AND p.IsActive = 1;
end
else -- If #IsDefaultProductKey is null
Select #ret = 1
If (#IsActive = 0) -- If Product is made inactive, make sure that its not a defaultkey for any product.
Begin
SELECT #ret = 0
FROM [Products] p
WHERE p.IsDefaultProductKey = #ProductId
End
IF (#ret IS NULL)
SET #ret = 0;
RETURN #ret;
END;
END;
--Select dbo.CheckProduct (2,1,0)
GO
ALTER TABLE [Products]
ADD CONSTRAINT chkActiveProduct
CHECK (dbo.CheckProduct(IsDefaultProductKey,ProductId, IsActive)=1);
go