SQL Server check constraint failing on correct input - sql-server

Question

I'm using a check constraint on a table to restrict what values are inserted in the table..
Here's an explanation of what I'm trying to do
If any Product(sedan) is associated to a specific ObjLevel (Toyota) then the same Product cannot be associated to another specific ObjLevel (Lexus)
After I apply the check constraint on the table, any insert containing ObjLevel "toyota" or "lexus" fails..
create table ObjLevel(
OLID int identity,
Name varchar(50) not null
)
insert into ObjLevel values('Ford')
insert into ObjLevel values('Toyota')
insert into ObjLevel values('Lexus')
insert into ObjLevel values('GM')
insert into ObjLevel values('Infiniti')
create table ObjInstance(
OLIID int identity (20,1),
OLID int
)
insert into ObjInstance values(1)
insert into ObjInstance values(2)
insert into ObjInstance values(3)
insert into ObjInstance values(4)
insert into ObjInstance values(5)
create table Product(
PID int identity(50,1),
Name varchar(20)
)
insert into Product values ('sedan')
insert into Product values ('coupe')
insert into Product values ('hatchback')
create table ObjInstanceProd(
OLIID int,
PID int
)
create FUNCTION [dbo].[fnObjProd] (#Pid int) RETURNS bit WITH EXECUTE AS CALLER AS
BEGIN
DECLARE #rv bit
DECLARE #cnt int
SET #cnt = 0
SET #rv = 0
SET #cnt=
(Select Count(*) from ObjInstanceProd olip
join ObjInstance oli
on olip.OLIID = oli.OLIID
join ObjLevel ol
on ol.OLID = oli.OLID
where ol.Name in ('Toyota','Lexus')
and PID = #Pid)
if(#cnt>0)
SET #rv = 1
RETURN #rv
END
ALTER TABLE [dbo].[ObjInstanceProd] WITH CHECK ADD CONSTRAINT [CK_OLIP] CHECK ([dbo].[fnObjProd]([PID])=0)
--Insert Statement
insert into ObjInstanceProd(OLIID,PID) values (22,51)
Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the CHECK constraint "CK_OLIP". The conflict occurred in database "tmp", table "dbo.ObjInstanceProd", column 'PID'.
The statement has been terminated.
--Execute Function
select [dbo].[fnObjProd] (51)
0
Initially the Table ObjInstanceProd is empty.. So, no matter what value I put in the table, as long as the function in the constraint returns a 0, it should accept it.. But it does not..
The function is correctly returning a 0 (when executed independently), but for some reason, the check constraint returns a 1

Answer 1

When the CHECK constraint fires, the row is already in the table. Therefore, the function is called, and since there is a row returned by the query, the function returns 1, not 0. Try this. Drop the constraint, insert your row successfully, and then run this query:
SELECT OLIID, PID, dbo.fnObjProd([PID]) FROM dbo.ObjInstanceProd;
It should return 1 for every value of PID. Try to add the constraint now. It will fail for the same reason.
Have you considered using a trigger for this? If you use a check constraint, this will turn any multi-row insert or update into a cursor behind the scenes. This can absolutely kill performance and concurrency depending on how you touch your tables. Here is a simple INSTEAD OF INSERT trigger to prevent bad values going in with a single operation, even for a multi-row insert:
CREATE TRIGGER dbo.trObjProd
ON dbo.ObjInstanceProd
INSTEAD OF INSERT AS
BEGIN
SET NOCOUNT ON;
IF NOT EXISTS
(
SELECT 1 FROM inserted
WHERE EXISTS
(
SELECT 1
FROM dbo.ObjInstanceProd AS olip
INNER JOIN dbo.ObjInstance AS oli
ON olip.OLIID = oli.OLIID
INNER JOIN dbo.ObjLevel AS ol
ON ol.OLID = oli.OLID
WHERE
ol.Name in ('Toyota','Lexus')
AND olip.PID = inserted.PID
)
)
BEGIN
INSERT ObjInstanceProd(OLIID, PID)
SELECT OLIID, PID FROM inserted;
END
ELSE
BEGIN
RAISERROR('At least one value was not good.', 11, 1);
SELECT OLIID, PID FROM inserted;
END
END
GO
If you're going to stick with a function, this is a much more efficient approach, however you need to define a way to determine that the current row being inserted is excluded from the check - I couldn't determine how to do that because there are no constraints on dbo.ObjInstanceProd. Is OLIID, PID unique?
ALTER FUNCTION [dbo].[fnObjProd]
(
#Pid INT
)
RETURNS BIT
WITH EXECUTE AS CALLER
AS
BEGIN
RETURN
(
SELECT CASE WHEN EXISTS
(
SELECT 1
FROM dbo.ObjInstanceProd AS olip
INNER JOIN dbo.ObjInstance AS oli
ON olip.OLIID = oli.OLIID
INNER JOIN dbo.ObjLevel AS ol
ON ol.OLID = oli.OLID
WHERE
ol.Name in ('Toyota','Lexus')
AND olip.PID = #Pid
) THEN 1 ELSE 0 END
);
END
GO