Hi,
I have working on this for God alone knows how many days. Can anyone please help me with how to correct this. My triggers are getting created successfully. However, when I try to insert/update I get the following errors. Please help!! I have a deadline in 24 hours!!
Thanks.

CREATE OR REPLACE TRIGGER trig1
BEFORE INSERT OR UPDATE OF hours ON Works_On
FOR EACH ROW
DECLARE
total_hrs Number(6,2);
lesshrs EXCEPTION;
morehrs EXCEPTION;
nullhrs EXCEPTION;
BEGIN
select sum(W.hours) into total_hrs
from Works_On W
group by W.essn;
IF (total_hrs) = null THEN
RAISE nullhrs;
ELSIF (total_hrs < 25 ) THEN
RAISE lesshrs;
ELSIF (total_hrs > 45) THEN
RAISE morehrs;
ELSE
dbms_output.put_line('Hours worked complies with company policy');
END IF;
EXCEPTION
WHEN lesshrs THEN
RAISE_APPLICATION_ERROR(-20001,'Total hours is less than 25 hours');
WHEN morehrs THEN
RAISE_APPLICATION_ERROR(-20002,'Total hours is more than 45 hours');
WHEN nullhrs THEN
RAISE_APPLICATION_ERROR(-20003,'Total hours is null');
END workd_on;
/

ERROR at line 1:
ORA-04098: trigger 'WORKD_ON1' is invalid and failed re-validation


CREATE OR REPLACE TRIGGER trig2
AFTER UPDATE OF salary ON Employee
FOR EACH ROW
DECLARE
less_sal Number;
more_sal Number
BEGIN

if(:new.salary < ld.salary) then
dbms_output.put_line('old salary is: '|| ' ' || ld.salary);
dbms_output.put_line('new salary is: '|| ' ' || :new.salary);
dbms_output.put_line('difference in salary is: '|| ' ' || ld.salary- :new.salary);
elsif(:new.salary > ld.salary) then
dbms_output.put_line('old salary is: '|| ' ' || ld.salary);
dbms_output.put_line('new salary is: '|| ' ' || :new.salary);
dbms_output.put_line('difference in salary is: '|| ' ' || :new.salary- ld.salary);
else
dbms_output.put_line('No change in salary');
end if;
END sal;
/
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error
ORA-06512: at "username.trig2", line 10
ORA-04088: error during execution of trigger 'username.trig2'

At a glance I would say you need to parenthesize the expression (:old.salary - :new.salary) otherwise PL/SQL will perform the string concatenation first and try to subtract :new.salary from the result, which would of course be a string.