Oracle SQL Trigger for View - sql

Question

I am having trouble with an SQL Trigger on a View. I have a table called Absence like so:
CREATE TABLE Absence
(
absence_id_pk varchar(6) NOT NULL,
staff_id_fk varchar(6),
start_date date,
end_date date,
reason varchar(30),
PRIMARY KEY (absence_id_pk),
FOREIGN KEY (staff_id_fk) REFERENCES Full_Time_Employee(staff_id_fk)
);
which records periods of staff absence from work.
Here is the problem! I would like to create a trigger that sends a message to the DBMS when a member of staff's total number of absent days is greater than 20 e.g. Eek! This staff is taking too much sick leave. In reality, it would probably be checked against a period of time e.g. in the last 6 months but this doesn't need to be that complex. Simply when the total of periods of absence is above 20 days on insert of a new record into the Absence table.
After reading some of the comments I have made this new trigger:
create or replace TRIGGER absence_check
BEFORE INSERT
ON absence
FOR EACH ROW
DECLARE
staffid absence.staff_id_fk%TYPE;
days number;
BEGIN
SELECT SUM(end_date - start_date) INTO days
FROM absence
WHERE staff_id_fk = staffid;
IF days > 20
THEN
DBMS_OUTPUT.PUT_LINE('Total days absent are more than 20' || staffid);
END IF;
END;
Any advice/guidance/solutions would be greatly appreciated! It would be a bonus if the message could print out the staff_id_fk that has just violated the > 20 days absent rule.
P.S. I am a University student and although this may be implemented in other ways, we have been asked to try and create triggers for our database scenario!

Answer 1

create or replace TRIGGER absence_check
BEFORE INSERT
ON absence
FOR EACH ROW
DECLARE
staffid absence.staff_id_fk%TYPE := :NEW.staff_id_fk;
days number;
BEGIN
SELECT SUM(end_date - start_date) INTO days
FROM Absence, Staff
WHERE Absence.staff_id_fk = staffid;
IF days > 20
THEN
DBMS_OUTPUT.PUT_LINE('Warning: Total number of days absent is more than 20 for staff member: ' || staffid);
END IF;
END;
Thanks for everyone's help and suggestions. It feels great when you finally crack a problem and I have learnt so much about Triggers by exploring this problem with your help.