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CHECK CONSTRAINT WITH USER DEFINED FUNCTION


-- HERE IS THE SIMPLE DEMONSTRATION CHECK CONSTRAINT WITH USER DEFINED FUNCTION.

-- SENARIO : ONLY ONE STUDENT CAN JOIN THE AT A TIME . ALL STUDENTS CAN WAIT UNTILL THE JOINED STUDENT CAN INACTIVE.

-- CREATE FUNCTION
CREATE FUNCTION CHECK_StudentJoinSQLClass(@JOINED BIT)
RETURNS BIT
AS
BEGIN
IF(@JOINED=1 AND (SELECT COUNT(*) FROM StudentJoinSQLClass WHERE  JOINED=@JOINED)>1)
BEGIN
                 SET @JOINED=0 --FALSE
END
ELSE
BEGIN
                SET @JOINED=1 --TRUE
END
RETURN @JOINED
END


-- CREATE TABLE
 CREATE TABLE StudentJoinSQLClass
(
 ID INT IDENTITY(1,1),
 NAME VARCHAR(10),
 JOINED BIT  NOT NULL CHECK(DBO.CHECK_StudentJoinSQLClass(JOINED)=1)
)

--INSERT DATA
INSERT INTO StudentJoinSQLClass(NAME,JOINED) VALUES('RAKESH',0)

INSERT INTO StudentJoinSQLClass(NAME,JOINED) VALUES('RAVI',1)

--HERE WE ARE INSERTING RAJU RECORD WITH TRUE VALUE .IN THIS CASE WE GET ERROR ,BEACUSE OF WE HAVE ALREADY RAVI RECORD WITH TRUE VALUE.
INSERT INTO StudentJoinSQLClass(NAME,JOINED) VALUES('RAJU',1)

-- UPDATE RAKESH RECORD FALSE TO TRUE.HERE ALSO WE GET THE ERROR.
UPDATE StudentJoinSQLClass SET JOINED=0 WHERE ID=1

--HERE WE UPDATING THE RAVI RECORD TO FALSE.
UPDATE StudentJoinSQLClass SET JOINED=0 WHERE ID=2

--TRY TO INSERT RAVI RECORD ONCE AGAIN.HERE THE VALUE IS INSERTED SUCESSFULLY.BEACAUSE NO TRUE RECORDS EXISTED IN THE TABLE RIGTH NOW.
INSERT INTO StudentJoinSQLClass(NAME,JOINED) VALUES('RAJU',1)


SELECT * FROM StudentJoinSQLClass


DROP TABLE StudentJoinSQLClass

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