How I can make the trigger calculates the sum of a column for specific condition

[Robin vanpersie]

I have table employees as following

CREATE TABLE `employees`
(
  `Employee_Id` Int NOT NULL AUTO_INCREMENT,
  `Employeer_Name` Char(150) NOT NULL,
  `Job` Varchar(100),
  `Salary` Decimal(10,2),
  PRIMARY KEY (`Employee_Id`)
) ENGINE = InnoDB
 AUTO_INCREMENT = 1
 ROW_FORMAT = Compact
;

During a month the employee can borrow some money from his salary many times,
it's required to record that amount of money borrowed , the remaining money from his monthly salary each time.

So I created table Salary_Transaction to record the transactions of borrowing and receiving money

CREATE TABLE `Salary_Transaction`
( `Salary_Transaction_Id` Int NOT NULL AUTO_INCREMENT,
  `money_amount` Decimal(10,2) NOT NULL,
  `Employee_Id` Int,
  PRIMARY KEY (`Salary_Transaction_Id`) );

And I created a table called debt_alerts to display the remaining money from the salary

CREATE TABLE `debt_alerts`
(
  `Debt_alert_id` Int NOT NULL AUTO_INCREMENT,
  `Salary_Transaction_Id` Int,
  `Remaining_money` Decimal(10,2),
  `Employee_Id` Int,
  PRIMARY KEY (`Debt_alert_id`)
) ENGINE = InnoDB
 AUTO_INCREMENT = 5
 ROW_FORMAT = Compact
;

--These are my indexes relationships

CREATE INDEX `IX_Relationship23` USING BTREE ON `debt_alerts` (`Salary_Transaction_Id`)
;

CREATE INDEX `IX_Relationship33` ON `debt_alerts` (`Employee_Id`)
;

CREATE INDEX `IX_Relationship11` USING BTREE ON `salary_transaction` (`Employee_Id`)
;
ALTER TABLE `salary_transaction` ADD CONSTRAINT `Relationship11` FOREIGN KEY (`Employee_Id`) REFERENCES `employees` (`Employee_Id`) ON DELETE CASCADE ON UPDATE CASCADE
;


ALTER TABLE `debt_alerts` ADD CONSTRAINT `Relationship23` FOREIGN KEY (`Salary_Transaction_Id`) REFERENCES `salary_transaction` (`Salary_Transaction_Id`) ON DELETE CASCADE ON UPDATE CASCADE
;

ALTER TABLE `debt_alerts` ADD CONSTRAINT `Relationship18` FOREIGN KEY (`Employee_Id`) REFERENCES `employees` (`Employee_Id`) ON DELETE CASCADE ON UPDATE CASCADE
;

For accomplishment of my task I created this trigger

DELIMITER $$
CREATE trigger Remaining_money_calc
AFTER INSERT ON Salary_Transaction 
FOR EACH ROW 
BEGIN
INSERT INTO debt_alerts(Remaining_money,Employee_Id,Salary_Transaction_Id) 
SELECT y.y-x.x,NEW.Employee_Id ,NEW.Salary_Transaction_Id
FROM ( SELECT SUM(money_amount) x
       FROM Salary_Transaction 
       WHERE Salary_Transaction_Id =  Salary_Transaction_Id ) x
JOIN ( SELECT Employees.Salary y 
       FROM Employees
       WHERE NEW.Employee_Id=Employees.Employee_Id 
       LIMIT 1 ) y;
END
$$ DELIMITER ;

The problem this trigger calculate the sum of all remaining_money in the debt_alerts table, I want it to do that for specific employee only.