Ensure one row only is set as default

Question

We have a Customers table. The default customer is 'CASH SALE'. Our business rules require one and one only customer be the default customer, but the default customer can be changed from CASH SALE to any other customer. Our business rules do not allow the
CASH SALE customer to be modified beyond setting the IsDefault attribute to true or false
There is a column IsDefault bit and a unique filtered index on IsDefault.
CREATE UNIQUE INDEX IX_Customers_IsDefault
ON Customers(IsDefault)
WHERE IsDefault = 1
This works to the extent that it does not allow more than 1 row to be marked IsDefault, but it does not ensure that there is one row marked IsDefault = 1, ie it is possible for IsDefault = 0 for all rows.
We also have constraints that prevent updating or deleting CASH SALE
-- the CASH SALE customer cannot be deleted
CREATE TRIGGER Customers_No_Delete ON Customers
INSTEAD OF DELETE
AS
IF ( SELECT CustomerId
FROM deleted
) = 0
BEGIN
RAISERROR ('The CASH SALE customer cannot be changed nor deleted.', 16, 1)
END
ELSE
BEGIN
DELETE FROM Customers
WHERE CustomerId = ( SELECT CustomerId
FROM deleted
END
GO
-- the CASH SALE customer cannot be changed
CREATE TRIGGER Customers_No_Update ON Customers
INSTEAD OF UPDATE
AS
IF ( SELECT CustomerId
FROM inserted
) = 0
BEGIN
RAISERROR ('The CASH SALE customer cannot be changed nor deleted.', 16, 1)
END
ELSE
BEGIN
DELETE FROM Customers
WHERE CustomerId = ( SELECT CustomerId
FROM deleted
END
GO
However the trigger Customers_No_Update also prevents updating the IsDefault column. I know this is a common business scenario:CASH SALE cannot be edited but another customer can be marked as the default customer, so I assume there is a standard solution
... if someone could please point me the way.

Answer 1

Thank you Erland. By using both of the triggers you suggested the problem is solved. I changed the trigger dont_touch_CASH_SALE so that rather than check if each column is updated, it checks that IsDefault is not the column being updated, ie IF NOT UPDATE(IsDefault)
Thanks again to everyone for your assistance.
Refering to your comment below
I may be mistaken but it appears the solution proposed by  Visakh16 does not allow IsDefault = 1 for a customer other than CASH SALE
I dont think thats true
See this illustration
--Sample table for illustration
CREATE TABLE Customers
ID int IDENTITY(1,1),
CustName varchar(100),
IsDefault bit
--Some sample data inserted
INSERT Customers (CustName,IsDefault)
VALUES
('Cust1',0),
('Cust2',0),
('Cust3',0),
('CASH SALE',1)
--The suggested trigger
CREATE TRIGGER Customers_No_Update ON Customers
INSTEAD OF UPDATE
AS
IF EXISTS( SELECT 1
FROM inserted i
INNER JOIN Deleted d
ON d.ID = i.ID
AND d.CustName <> COALESCE(i.CustName,'')
WHERE d.CustName = 'CASH SALE'
BEGIN
RAISERROR ('The CASH SALE customer cannot be changed nor deleted.', 16, 1)
END
ELSE
BEGIN
UPDATE c
SET CustName = i.CustName,
IsDefault = i.IsDefault
FROM Customers c
INNER JOIN Inserted i
ON i.ID = c.ID
END
GO
--The updates
-- Modifying CASH SALE IsDefault. This will succeed
UPDATE Customers
SET IsDefault = 0
WHERE CustName = 'CASH SALE'
-- Modifying Cust1 IsDefault. This will succeed
UPDATE Customers
SET IsDefault = 1
WHERE CustName = 'Cust1'
-- Modifying Cust2 Name. This will succeed
UPDATE Customers
SET CustName = 'Cust2 Test'
WHERE CustName = 'Cust2'
-- Modifying CASH SALE Name. This will fail
UPDATE Customers
SET CustName = 'CASH SALE Test'
WHERE CustName = 'CASH SALE'
Only thing I've have not covered is the scenario which ensures you've only single default cust value at any point of time. If you want that to be enforced just extend trigger as below
CREATE TRIGGER Customers_No_Update ON Customers
    INSTEAD OF UPDATE
AS
    IF EXISTS( SELECT 1         
         FROM   inserted i
         INNER JOIN Deleted d
         ON d.ID = i.ID
         AND d.CustName <> COALESCE(i.CustName,'')
         WHERE d.CustName = 'CASH SALE'
        BEGIN
            RAISERROR ('The CASH SALE customer cannot be changed nor deleted.', 16, 1)    
        END
     ELSE IF EXISTS( SELECT 1         
         FROM   Customers i
         WHERE IsDefault = 1
         AND NOT EXISTS (SELECT 1 FROM deleted WHERE CustName = i.CustName)
       BEGIN
            RAISERROR ('There cant be more than 1 default customers.', 16, 1)    
        END  
    ELSE
        BEGIN
            UPDATE c
            SET CustName = i.CustName,
            IsDefault = i.IsDefault
            FROM Customers c
            INNER JOIN Inserted i
            ON i.ID = c.ID
        END
GO
and do updates
Please Mark This As Answer if it helps to solve the issue Visakh ---------------------------- http://visakhm.blogspot.com/ https://www.facebook.com/VmBlogs