PL/SQL Trigger to calculate total attendance - oracle

Question

I want to create a trigger to calculate total attendance for each distinct record from the given table.
DAILY_ATT
The second table should generate output like this:
TOTAL_ATT
Here's what I have done:
CREATE TABLE daily_att
(
roll_no NUMBER(5),
subject VARCHAR2(10),
attendance NUMBER(5),
date_att DATE
);
CREATE TABLE total_att
(
roll_no NUMBER(5) NOT NULL PRIMARY KEY,
total_attendance NUMBER(5)
);
INSERT INTO DAILY_ATT VALUES(1, 'MATHS', 0, '04-MAY-18');
INSERT INTO DAILY_ATT VALUES(1, 'ENG', 1, '04-MAY-18');
INSERT INTO DAILY_ATT VALUES(2, 'MATHS', 1, '04-MAY-18');
INSERT INTO DAILY_ATT VALUES(2, 'ENG', 1, '04-MAY-18');
INSERT INTO DAILY_ATT VALUES(1, 'MATHS', 1, '05-MAY-18');
INSERT INTO DAILY_ATT VALUES(1, 'ENG', 1, '05-MAY-18');
INSERT INTO DAILY_ATT VALUES(2, 'MATHS', 0, '05-MAY-18');
INSERT INTO DAILY_ATT VALUES(2, 'ENG', 0, '05-MAY-18');
SELECT * FROM DAILY_ATT;
CREATE OR replace TRIGGER att
AFTER INSERT OR UPDATE ON daily_att
FOR EACH ROW
BEGIN
SELECT SUM(attendance)
INTO Total_att(total_attendance)
FROM daily_att
WHERE roll_no = :NEW.roll_no;
END;

Answer 1

Hope this will serve your purpose
CREATE OR REPLACE TRIGGER create_subtotal
AFTER INSERT OR UPDATE OR DELETE ON DAILY_ATTENDANCE
FOR EACH ROW
DECLARE
V_ROLL_NO NUMBER(8);
V_TOT_ATTENDANCE NUMBER(4);
BEGIN
SELECT ROLL_NO,SUM(ATTENDANCE)
INTO V_ROLL_NO,V_TOT_ATTENDANCE
FROM DAILY_ATTENDANCE
WHERE ROLL_NO = :new.ROLL_NO AND DATE_ATT=TRUNC(SYSDATE);
BEGIN
update TOTAL_ATTENDANCE
set TOTAL_ATTENDANCE= TOT_ATTENDANCE
where ROLL_NO = V_ROLL_NO;
if sql%rowcount = 0 then
-- no rows were updated, so the record does not exist
insert into TOTAL_ATTENDANCE (ROLL_NO,TOTAL_ATTENDANCE )
values ( V_ROLL_NO,V_TOT_ATTENDANCE );
END IF;
END;
END;

Answer 2

I would recommend you to not abuse a Trigger for this purpose. Simply create a view.
CREATE
OR replace VIEW total_att AS
SELECT roll_no
,SUM(attendance) as total_attendance
FROM daily_att
GROUP BY roll_no;
select * FROM total_attendance;
ROLL_NO TOTAL_ATTENDANCE
1 3
2 2

Answer 3

My suggestion would be to use this set of 2 triggers:
The core one:
CREATE OR REPLACE TRIGGER create_subtotal
AFTER UPDATE OF roll_no, attendance OR INSERT OR DELETE ON daily_att
FOR EACH ROW
DECLARE
v_roll_no daily_att.roll_no%TYPE;
v_before daily_att.attendance%TYPE;
v_after daily_att.attendance%TYPE;
v_diff daily_att.attendance%TYPE;
BEGIN
IF UPDATING AND (:NEW.roll_no <> :OLD.roll_no) THEN
RAISE_APPLICATION_ERROR( -20001, 'Altering ROLL_NO is not allowed!' );
END IF;
IF NOT INSERTING THEN
v_before := :OLD.attendance;
v_roll_no := :OLD.roll_no;
ELSE
v_before := 0;
v_roll_no := :NEW.roll_no;
END IF;
IF NOT DELETING THEN
v_after := :NEW.attendance;
ELSE
v_after := 0;
END IF;
v_diff := v_after - v_before;
IF INSERTING OR (v_diff <> 0) THEN
UPDATE total_att
SET total_attendance = total_attendance + v_diff
WHERE roll_no = v_roll_no;
IF SQL%ROWCOUNT = 0 THEN
INSERT INTO total_att (roll_no, total_attendance)
VALUES (v_roll_no, v_diff);
END IF;
END IF;
END;
An auxiliary one:
CREATE OR REPLACE TRIGGER delete_subtotal
AFTER DELETE ON daily_att
BEGIN
DELETE FROM total_att
WHERE NOT EXISTS (SELECT 1
FROM daily_att d
WHERE total_att.roll_no = d.roll_no);
END;
I've assumed that ROLL_NO is never NULL, nor should ever be changed, and that ATTENDANCE is also never NULL. I basically ignored the SUBJECT and DATE_ATT columns, as from the question, they do not seem to impact the goal.
The auxiliary trigger can be dropped, if you do not need to handle deletes from the DAILY_ATT table, or can stand zero entries in TOTAL_ATT table, for things removed from DAILY_ATT completely.
For better performance, the check for ROLL_NO change should be moved to a separate BEFORE UPDATE trigger. The auxiliary trigger will usually benefit much from an index on ROLL_NO in the DAILY_ATT table.
Here's an SQL Fiddle for this.