Count Distinct is not working in UDF in SQL Server 2008 - sql-server-2008

Question

I am facing problem in creating UDF with count distinct in query. I have created a table amtest which have the following :
CREATE TABLE [dbo].[amtest](
[ztime] [datetime] NULL,
[zutime] [datetime] NULL,
[zid] [int] NOT NULL,
[xamacadyr] [int] NOT NULL,
[xamreg] [varchar](20) NOT NULL,
[xmobile] [varchar](15) NULL,
[xamclasssec] [varchar](30) NULL,
PRIMARY KEY CLUSTERED
(
[zid] ASC,
[xamacadyr] ASC,
[xamreg] ASC
)
Data in the amtest table are as follows:
zid xamacadyr xamreg xmobile xamclasssec
100000 2013 201508001 01713236075 Section-B
100000 2014 201508003 01713236072 Section-A
100000 2015 201508001 01713236071 Section-A
100000 2015 201508003 01713236073 Section-A
100000 2015 201508004 01713236074 Section-A
Now I have created an User-defined-functions with parameter xamreg so that, for any xamreg value, there can not be 2 distinct value in xamclasssec field. For example, here for 201508001 value in xamreg, there should not be 2 distinct values (Section-A, Section-B), it should only allow either Section-A or Section-B. For that I have created the following UDF:
ALTER FUNCTION [dbo].[CheckTest](#xamreg varchar(30))
RETURNS int
AS
BEGIN
DECLARE #retvar int;
select #retvar = COUNT(*) from (select xamclasssec from amtest where xamreg=#xamreg group by xamclasssec) as tbl ;
RETURN #retvar
END
And I have created a constraint for amtest table, which is as follows:
alter table amtest
add constraint chkAmtest
check(dbo.CheckTest(xamreg)<=1)
Where I have made a mistake, as it is not working?

Answer 1

First of all your function can be simplified to:
CREATE FUNCTION [dbo].[CheckTest](#xamreg varchar(30))
RETURNS INT
AS
BEGIN
RETURN (SELECT COUNT(DISTINCT xamclasssec) FROM amtest WHERE xamreg = #xamreg);
END
GO
I run your query with SQL Fiddle it seems to work but only with INSERT:
SqlFiddleDemo
If I try to set DISTINCT values you get:
The INSERT statement conflicted with the CHECK constraint "chkAmtest".
The conflict occurred in database "db_3_5ec42", table "dbo.amtest",
column 'xamreg'.
But there is one drawback with your solution, it won't work when UPDATE occurs.
SqlFiddleDemo2
INSERT INTO [dbo].[amtest]
(zid ,xamacadyr , xamreg , xmobile , xamclasssec)
VALUES (100000 , 2013 , 201508001 , 01713236075 , 'Section-B'),
(100000, 2015, 201508001 , 01713236071 , 'Section-B');
UPDATE [dbo].[amtest]
SET xamclasssec = 'Section-A'
WHERE xmobile = 01713236071;
SELECT *
FROM [dbo].[amtest];
Use AFTER TRIGGER instead:
SqlFiddleDemo3
CREATE TRIGGER trg_amtest
ON [dbo].[amtest]
AFTER INSERT, UPDATE
AS
IF EXISTS (SELECT xamreg, COUNT(DISTINCT xamclasssec)
FROM amtest
GROUP BY xamreg
HAVING COUNT(DISTINCT xamclasssec) > 1)
ROLLBACK;
GO