Check Constraint not firing correctly - sql-server

Question

I'm pretty sure I am doing something wrong, as this is my first check constraint, but I can't understand why it's not working. I need to check that a date range doesn't overlap.
ALTER FUNCTION fn_DateOverlaps (#StartDate DATE, #EndDate DATE, #ProjectID INT)
RETURNS BIT
AS
BEGIN
DECLARE #Ret BIT
SET #Ret = 1
IF NOT EXISTS(
SELECT * FROM project_sprint
WHERE ((#StartDate >= StartDate AND #EndDate <= EndDate)
OR (#StartDate <= StartDate AND #EndDate >= EndDate))
AND ProjectId = #ProjectId
)
BEGIN
SET #Ret = 0
END
RETURN #Ret
END
GO
I then apply this to my table:
ALTER TABLE Project_Sprint WITH CHECK ADD CONSTRAINT ck_DateOverlaps CHECK (dbo.fn_DateOverlaps([StartDate], [EndDate], [ProjectId])=1)
GO
When I test the function, I get a good result:
SELECT dbo.fn_DateOverlaps('2013-06-10', '2013-06-13', 1)
But then when I apply the same date range and project ID to my table, it allows the insert. It should fail it.
What am I doing wrong?

Answer 1

If you change SELECT * FROM project_sprint to SELECT * FROM dbo.project_sprint the function will correctly evaluate, but you won't get the desired behavior, since the value you are inserting or editing will lead to an unwanted find.
To prevent this you will have to add an additional ID field to except the row you want to edit/insert for the check.
Create Table Project_Sprint(ID int IDENTITY(1,1) NOT NULL,ProjectID int,StartDate DateTime,EndDate DateTime)
go
Alter FUNCTION fn_DateOverlaps (#ID int,#StartDate DATE, #EndDate DATE, #ProjectID INT)
RETURNS BIT
AS
BEGIN
DECLARE #Ret BIT
SET #Ret = 0
IF NOT EXISTS(
SELECT * FROM dbo.project_sprint
WHERE
#ID<>ID AND
((#StartDate >= StartDate AND #EndDate <= EndDate)
OR (#StartDate <= StartDate AND #EndDate >= EndDate))
AND ProjectId = #ProjectId
)
BEGIN
SET #Ret = 1
END
RETURN #Ret
END
GO
ALTER TABLE Project_Sprint ADD CONSTRAINT ck_DateOverlaps CHECK (dbo.fn_DateOverlaps([ID],[StartDate], [EndDate], [ProjectId])=1)