How to create a trigger for this situation? - sql

Question

I have a problem inserting values into a Class table.
I want to write a trigger to prevent happening "an instructor teaches in different class_Id at the same time".
How can I do this?
CREATE TABLE Class
(
Class_ID BIGINT,
c_InstrumentID BIGINT NOT NULL,
c_StudentID BIGINT,
c_InstructorID BIGINT NOT NULL,
c_InstituteId BIGINT NOT NULL,
c_TermSeason NVARCHAR(10),
c_TermYear INT,
c_TimeOfClass TIME NOT NULL,
c_DayOfClass NVARCHAR(30),
c_Eligibility INT,
c_RemainingSession INT,
CONSTRAINT cons_Season
CHECK(c_TermSeason IN ('Spring', 'Summer', 'Fall', 'Winter')),
CONSTRAINT cons_TimeClass
CHECK(c_TimeOfClass BETWEEN '08:30:00' AND '20:30:00'),
CONSTRAINT cons_RemainSession
CHECK (c_RemainingSession BETWEEN 0 AND 12),
FOREIGN KEY(c_InstrumentID)
REFERENCES Instrument(Instrument_ID) ON DELETE NO ACTION,
FOREIGN KEY(c_StudentID)
REFERENCES Student(Student_ID) ON DELETE NO ACTION,
FOREIGN KEY(c_InstructorID)
REFERENCES Instructor(Instructor_ID) ON DELETE NO ACTION,
FOREIGN KEY(c_InstituteId)
REFERENCES Institute(Institute_ID) ON DELETE NO ACTION,
PRIMARY KEY (Class_ID)
)
This is the trigger which I've created:
CREATE OR ALTER TRIGGER One_InstructorDuplicate
ON Class
AFTER INSERT
AS
BEGIN
IF (NOT EXISTS (SELECT *
FROM Class C, ((SELECT * FROM CLASS)
EXCEPT (SELECT * FROM inserted)) AS newC
WHERE newC.c_InstructorID = C.c_InstructorID
AND newC.c_DayOfClass != C.c_DayOfClass
AND newC.c_TermSeason != C.c_TermSeason
AND newC.c_TermYear != C.c_TermYear
AND newC.c_TimeOfClass != C.c_TimeOfClass))
ROLLBACK TRAN
END;

Answer 1

Use inserted and JOIN to the Class table. Check for existence of rows in table that matches your requirement (c_DayOfClass, c_TermSeason etc)
CREATE OR ALTER TRIGGER One_InstructorDuplicate
ON Class
AFTER INSERT
AS
BEGIN
IF EXISTS
(
SELECT *
FROM inserted i
INNER JOIN Class c ON i.c_InstructorID = c.c_InstructorID
WHERE i.Class_ID <> c.Class_ID
AND i.c_DayOfClass = c.c_DayOfClass
AND i.c_TermSeason = c.c_TermSeason
AND i.c_TermYear = c.c_TermYear
AND i.c_TimeOfClass = c.c_TimeOfClass
)
BEGIN
ROLLBACK TRAN
END
END;