Update Trigger : Supertype/subtypes tables - sql-server-2008

Question

I need some help about triggers. I’m currently developing a platform and a database in order to manage exams at my university. Here is my problem:
I have 1 supertype table, which contains all the persons registered on the platform. I have to be able to make the distinction for each person between the functions “Candidate” and “Examiner”. So I have my 2 subtype tables, one for all the candidates and one for all the examiners. To achieve that, I’m using insert triggers.
In addition, a person can be both Candidate and Examiner, but not at the same time. So after updating the supertype table, I also need a trigger to be able to delete a specific row from one of the two-subtype table and insert the user information on the other.
Here is a simplified design of these 3 tables:
My INSERT trigger :
ALTER TRIGGER [dbo].[role_insert]
ON [dbo].[alemp_persons]
FOR INSERT
AS
DECLARE #random_number int
SELECT #random_number = CAST(CAST(rand() as binary(2)) as int)
BEGIN
INSERT INTO dbo.alemp_candidates
(
id_person, random_number
)
SELECT id_person, # random_number
FROM INSERTED
WHERE function='Candidate'
INSERT INTO dbo.alemp_examiners
(
id_person
)
SELECT id_person
FROM INSERTED
Where function='Examiner'
END
GO
My UPDATE trigger :
ALTER TRIGGER [dbo].[role_update] ON [dbo].[alemp_persons]
AFTER UPDATE
AS
DECLARE #id_person int
DECLARE #newFunction int SELECT #newFunction=function FROM inserted
DECLARE #random_number int SELECT # random_number = CAST(CAST(rand() as binary(2)) as int)
IF #newFunction = 'Candidate'
BEGIN
DELETE
FROM dbo.alemp_examiners
WHERE id_person=#id_person
END
BEGIN
SET IDENTITY_INSERT dbo.alemp_candidates ON;
INSERT INTO dbo.alemp_candidates
(
id_person, random_number
)
SELECT #id_person, random_number
SET IDENTITY_INSERT dbo.alemp_candidates OFF;
END
IF #newFunction = 'Examiner'
BEGIN
DELETE
FROM dbo.alemp_candidates
WHERE id_person=#id_person
END
BEGIN
SET IDENTITY_INSERT dbo.alemp_examiners ON;
INSERT INTO dbo.alemp_examiners
(
id_person
)
SELECT #id_person
SET IDENTITY_INSERT dbo.alemp_examiners Off;
END
GO
As I said above, my INSERT trigger works as I want. However when I want to update the function of one person, I got an error :
Explicit value must be specified for identity column either when IDENTITY_INSERT is set
to ON or when a replication user is inserting into a NOT FOR REPLICATION identity column.

Answer 1

Some simple notes:
1) You should follow Mitch Wheat's advice and rewrite these triggers because inserted and deleted tables could have more than one row. For example, your trigger will have a bad behavior when is executed the next statement UPDATE [dbo].[alemp_persons] SET function = CASE WHEN id_person = 1 THEN 'Candidate' ELSE 'Examiner' END WHERE id_person IN (1,2) if the first person's function is 'Examiner' and the second person's function is 'Candidate'.
2) [dbo].[alemp_persons].function's data type should be [tiny]int or char(1) and not varchar(something greater than 1) (Where function='Examiner').
3) [dbo].[alemp_persons].function column should disallow Nulls.
4) [dbo].[alemp_persons].function column should has a CHECK constraint:
ALTER TABLE [dbo].[alemp_persons]
ADD CONSTRAINT CK_alemp_persons_function_Verify CHECK ( function IN ('Candidate', 'Examiner') );
5) It would be nice to add
a function column to [dbo].[alemp_candidates] and [dbo].[alemp_examiners] tables,
two check constraints on [dbo].[alemp_candidates] (function = 'Candidate') and [dbo].[alemp_examiners] (function = 'Examiner'),
an UNIQUE index on [dbo].[alemp_persons](id_person, function),
two FKs between [dbo].[alemp_candidates/examiners](id_person, function) and [dbo].[alemp_persons](id_person, function).
This way, you can be sure that [dbo].[alemp_candidates] table has only candidates and [dbo].[alemp_examiners] has only examiners and every person can be only candidate or examiner at one time.
6) You should disallow IDENTITY property for id_person columns in [dbo].[alemp_candidates] and [dbo].[alemp_examiners] table (SET IDENTITY_INSERT dbo.alemp_candidates ...).
8) And this statement IF #newFunction = 'Candidate' should raise an error because #newFunction data type is 'INT'.
9) And the AFTER UPDATE trigger on [dbo].[alemp_persons] table will move data between candidates and examiners tables (not tested):
ALTER TRIGGER [dbo].[role_update]
ON [dbo].[alemp_persons]
FOR UPDATE
AS
BEGIN
DECLARE #selected_rows TABLE (
id_person INT PRIMARY KEY, -- or BIGINT, look at alemp_person.id_person data type
new_function VARCHAR(50) NOT NULL -- look at alemp_person.function column data type
);
INSERT #selected_rows (id_person, new_function)
SELECT new.id_person, new.function
FROM inserted as new INNER JOIN deleted as old ON new.id_person = old.id_person
WHERE new.function <> old.function;
MERGER dbo.alemp_candidates AS dest
USING #selected_rows AS src ON dest.id_person = src.id_person
WHEN MATCHED THEN
DELETE
WHEN NOT MATCHED BY TARGET AND src.new_function = 'Candidate' THEN
INSERT (id_person, random_number)
VALUES (src.id_person, CONVERT(BINARY(2), CHECKSUM(NEWID()));
MERGER dbo.alemp_examiners AS dest
USING #selected_rows AS src ON dest.id_person = src.id_person
WHEN MATCHED THEN
DELETE
WHEN NOT MATCHED BY TARGET AND src.new_function = 'Examiner' THEN
INSERT (id_person)
VALUES (src.id_person);
END