SQL: Can I write a CHECK constraint that validates data in a foreign table using a foreign key? - sql

Question

I am designing a test database in SQL Server 2008 R2 and/or SQL Azure. (All of my code will run on both, so far.)
I have a table with a foreign key, and I need to add a constraint that references a field in the foreign table.
Normally I would have the foreign table manage it's own validation checks, but there are cases where that is impossible (or illogical). I've provided some sample-code that displays what I am trying to accomplish.
CREATE TABLE CustomerOrder
(
ID INT NOT NULL IDENTITY PRIMARY KEY,
CustomerID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES Customer(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
ProductID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES Product(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
Quantity INT NOT NULL DEFAULT 1,
IsPaid BIT NOT NULL DEFAULT 0
)
GO
CREATE TABLE RMA
(
ID INT NOT NULL IDENTITY PRIMARY KEY,
CustomerOrderID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES CustomerOrder(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
-- Add constraint to prevent RMAs from being
-- created for orders that have not been paid.
-- This could be a column constraint, or a table constraint.
CHECK ( CustomerOrderID.IsPaid = 1 )
-- ERROR: 'The multi-part identifier "CustomerOrderID.IsPaid"
-- could not be bound.'
)
GO
In this example, it doesn't make sense to put the CHECK constraint in the CustomerOrder table, because a row in the CustomerOrder table is perfectly happy being unpaid as long as there are no RMAs for the order. Furthermore, a constraint in the CustomerOrder table would still need to reference the RMA table to confirm whether there is an RMA, so the same issue remains.
I've also tried:
CHECK (EXISTS(SELECT co.ID FROM CustomerOrder co
WHERE co.ID=CustomerOrderID AND
co.IsPaid=1))
-- ERROR: 'Subqueries are not allowed in this context.
-- Only scalar expressions are allowed.'
Since this is a static constraint for basic data validation, and will never be referenced by any other object, I'd like to avoid making this into a scalar function or stored procedure.
However, in order to avoid using a scalar function, I will need to define the constraint within SQL (preferrably at the same time my database is deployed and the table is created).
What SQL syntax could I use here to define this type of constraint?

Answer 1

Another option (since you asked for a solution without adding triggers or enlarging the foreign key), is removing the IsPaid column and adding another table for paid orders:
CREATE TABLE CustomerOrder
(
ID INT NOT NULL IDENTITY PRIMARY KEY,
CustomerID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES Customer(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
ProductID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES Product(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
Quantity INT NOT NULL DEFAULT 1
)
GO
CREATE TABLE CustomerOrderPaid
(
ID INT NOT NULL PRIMARY KEY
FOREIGN KEY REFERENCES CustomerOrder(ID)
ON DELETE NO ACTION ON UPDATE CASCADE
)
GO
CREATE TABLE RMA
(
ID INT NOT NULL IDENTITY PRIMARY KEY,
CustomerOrderID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES CustomerOrderPaid(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
)
GO