how to use triggers when comparing two data from different tables? [closed] - sql

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CONSTRAINT (Movie.Date – person.DateOfBirth >= Movie.MinAge)
I am trying to use triggers for this constraint, I am not sure how to use triggers when you are comparing different data from different tables.

Answer 1

Something like this.. I didn't test it!!
CREATE OR REPLACE TRIGGER check_movie_parent_rating
BEFORE INSERT OR UPDATE OF DateOfBirth ON Person
REFERENCING NEW AS n
FOR EACH ROW
DECLARE
v_allowed VARCHAR2(1);
BEGIN
BEGIN
SELECT CASE WHEN (Movie.Date - person.DateOfBirth >= Movie.MinAge)
THEN 'Y'
ELSE 'N'
END As Allowed
INTO v_allowed
FROM Movie , Person
WHERE Movie.id = :n.movieId
AND Person.movieId = Movie.id
AND Person.id = :n.id;
EXCEPTION
WHEN NO_DATA_FOUND THEN
v_allowed := NULL;
END;
IF v_allowed IS NULL THEN
Raise_application_error(-20202, 'Movie Detail not available in parent');
ELSIF v_allowed = 'N' THEN
Raise_application_error(-20201, 'Movie Restricted for User');
END IF;
END;
/

Answer 2

Here is a solution that will avoid the mutating table error that arises when you select from a table within a row level trigger defined against the same table.
You should check the constraint whenever any involved table is changed.
Ideally, there should also be serialisation locks at within the trigger to ensure transactions executing concurrently do not alter the data in such a way as to break the constraint. This can be achieved by using modules within the supplied DBMS_LOCK package.
CREATE OR REPLACE TRIGGER Movie_Check_Age
AFTER INSERT OR UPDATE OF <Join_Columns>, Date, MinAge ON Movie
FOR EACH ROW
DECLARE
CURSOR csrPersons
IS
SELECT NULL
FROM Person p
WHERE p.<Join_Columns> = :new.<Join_Columns>
AND (Months_Between(:new.Date, p.DateOfBirth) / 12) < :new.MinAge;
rPerson csrPersons%ROWTYPE;
BEGIN
OPEN csrPersons;
FETCH csrPersons INTO rPerson;
IF csrPersons%FOUND THEN
CLOSE csrPersons;
Raise_Application_Error(-20001, 'Person too young to see movie');
ELSE
CLOSE csrPersons;
END IF;
END;
/
CREATE OR REPLACE TRIGGER Person_Check_Age
AFTER INSERT OR UPDATE OF <Join_Columns>, DateOfBirth ON Person
FOR EACH ROW
DECLARE
CURSOR csrMovies
IS
SELECT NULL
FROM Movie m
WHERE m.<Join_Columns> = :new.<Join_Columns>
WHERE (Months_Between(m.Date, :new.DateOfBirth) / 12) < m.MinAge;
rMovie csrMovies%ROWTYPE;
BEGIN
OPEN csrMovies;
FETCH csrMovies INTO rMovie;
IF csrMovies%FOUND THEN
CLOSE csrMovies;
Raise_Application_Error(-20001, 'Person too young to see movie');
ELSE
CLOSE csrMovies;
END IF;
END;
/