Question

I want to add a check constraint to an already-existing table which does a check based upon a SELECT COUNT(*) statement. I know my SELECT statement but when I have created the DROP AND CREATE TO script I am unsure where to add my code and what format it should take?

Is there a way to do this in the GUI, or could someone show me an example of a CREATE table statement with a CHECK constraint, considering of a SELECT COUNT(*)?

I know that a CHECK constraint just has to evaluate to a boolean from here: http://msdn.microsoft.com/en-gb/library/ms188258(v=sql.105).aspx

Answer 1

The first thing of note is that you cannot directly use sub-queries in check constraints, but creating a function is the way around that. This will help you:

CREATE FUNCTION dbo.fnRedRows()
RETURNS INT
AS
BEGIN
    DECLARE @Return INT
    SELECT @Return=COUNT(*) FROM dbo.Red
    RETURN @Return
END
GO
CREATE TABLE dbo.Red 
(
    id INT IDENTITY(1,1), 
    test VARCHAR(max), 
    CONSTRAINT CK_MaxRows CHECK (dbo.fnRedRows()<5)
)
GO
INSERT INTO dbo.Red (test) VALUES ('HI')
INSERT INTO dbo.Red (test) VALUES ('The')
INSERT INTO dbo.Red (test) VALUES ('first four')
INSERT INTO dbo.Red (test) VALUES ('will work')
INSERT INTO dbo.Red (test) VALUES ('This one will fail')
GO
DROP TABLE dbo.Red
GO
DROP FUNCTION dbo.fnRedRows
GO