mysql trigger - update value, value= row count - mysql

Question

I want to run a mysql trigger to update value of table leave_taken col num, when a table called leave was updated, inserted or deleted.
Like if I insert this rows to leave:
INSERT INTO `leave` (`id_leave`, `staff_leave_application_id_staff_leave_application`, `staff_leave_application_staff_id_staff`, `leave_type_id_leave_type`, `date`, `active`, `date_updated`) VALUES
(7, 7, 6, 7, '2013-01-21', 1, '2013-01-18'),
(8, 7, 6, 7, '2013-01-22', 1, '2013-01-18'),
(9, 7, 6, 7, '2013-01-23', 1, '2013-01-18'),
(10, 7, 6, 7, '2013-01-24', 1, '2013-01-18'),
(11, 7, 6, 7, '2013-01-25', 2, '2013-01-18');
then table leave_taken col num value was updated to 5 where:
staff_leave_application_staff_id_staff = currently updated staff_leave_application_staff_id_staff,
and
leave_type_id_leave_type = currently updated leave_type_id_leave_type
I have included the Table below.
Table Leave
CREATE TABLE IF NOT EXISTS `leave` (
`id_leave` int(11) NOT NULL AUTO_INCREMENT,
`staff_leave_application_id_staff_leave_application` int(11) NOT NULL,
`staff_leave_application_staff_id_staff` int(11) NOT NULL,
`leave_type_id_leave_type` int(11) NOT NULL,
`date` date NOT NULL,
`active` int(11) NOT NULL DEFAULT '1',
`date_updated` date NOT NULL,
PRIMARY KEY (`id_leave`,`staff_leave_application_id_staff_leave_application`,`staff_leave_application_staff_id_staff`),
KEY `fk_table1_leave_type1` (`leave_type_id_leave_type`),
KEY `fk_table1_staff_leave_application1` (`staff_leave_application_id_staff_leave_application`,`staff_leave_application_staff_id_staff`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=12 ;
Table Leave taken:
CREATE TABLE IF NOT EXISTS `leave_balance` (
`id_leave_balance` int(11) NOT NULL AUTO_INCREMENT,
`staff_id_staff` int(11) NOT NULL,
`leave_type_id_leave_type` int(11) NOT NULL,
`balance` int(3) NOT NULL,
`date_added` date NOT NULL,
PRIMARY KEY (`id_leave_balance`),
UNIQUE KEY `id_leave_balance_UNIQUE` (`id_leave_balance`),
KEY `fk_leave_balance_staff1` (`staff_id_staff`),
KEY `fk_leave_balance_leave_type1` (`leave_type_id_leave_type`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;

Answer 1

First, define a unique key over the identifying columns in your leave table:
ALTER TABLE leave
ADD UNIQUE (staff_leave_application_staff_id_staff, leave_type_id_leave_type);
Then go ahead and define your triggers:
CREATE TRIGGER my_insert_trigger AFTER INSERT ON leave FOR EACH ROW
INSERT INTO leave_taken
(staff_leave_application_staff_id_staff, leave_type_id_leave_type, num)
VALUES
(NEW.staff_leave_application_staff_id_staff, NEW.leave_type_id_leave_type, 1)
ON DUPLICATE KEY UPDATE
num = num + 1;
CREATE TRIGGER my_delete_trigger AFTER DELETE ON leave FOR EACH ROW
UPDATE leave_taken
SET num = num - 1
WHERE staff_leave_application_staff_id_staff = OLD.staff_leave_application_staff_id_staff
AND leave_type_id_leave_type = OLD.leave_type_id_leave_type;
DELIMITER ;;
CREATE TRIGGER my_update_trigger AFTER UPDATE ON leave FOR EACH ROW
IF NOT (
NEW.staff_leave_application_staff_id_staff <=> OLD.staff_leave_application_staff_id_staff
AND NEW.leave_type_id_leave_type <=> OLD.leave_type_id_leave_type
) THEN
UPDATE leave_taken
SET num = num - 1
WHERE staff_leave_application_staff_id_staff = OLD.staff_leave_application_staff_id_staff
AND leave_type_id_leave_type = OLD.leave_type_id_leave_type;
INSERT INTO leave_taken
(staff_leave_application_staff_id_staff, leave_type_id_leave_type, num)
VALUES
(NEW.staff_leave_application_staff_id_staff, NEW.leave_type_id_leave_type, 1)
ON DUPLICATE KEY UPDATE
num = num + 1;
END IF;;
DELIMITER ;