Create trigger before insert in table - mysql

Question

I created a trigger before inserting into the employee table to calculate its age, but trying to insert into the employee table returns the following erro Error Code: 1442. Can't update table 'employee' in stored function/trigger because it is already used by statement which invoked this stored function/trigger
CREATE TABLE IF NOT EXISTS person (
ssn INT(20) NOT NULL,
name_person VARCHAR(200) NOT NULL,
birth_date DATE NOT NULL,
PRIMARY KEY (ssn))
ENGINE = InnoDB;
CREATE TABLE IF NOT EXISTS employee (
ssn_employee INT(20) NOT NULL,
job_title VARCHAR(100) NOT NULL,
age INT(10) DEFAULT NULL,
PRIMARY KEY(ssn_employee),
FOREIGN KEY (ssn_employee)
REFERENCES person (ssn)
ON UPDATE CASCADE)
ENGINE = InnoDB;
DELIMITER $$
CREATE TRIGGER employeeAge
BEFORE INSERT ON employee
FOR EACH ROW
BEGIN
DECLARE dob DATE;
DECLARE ssn_employee1 int;
SELECT new.ssn_employee INTO ssn_employee1;
SELECT person.birth_date into dob FROM person WHERE ssn = ssn_employee1;
UPDATE employee
SET new.age = DATEDIFF(dob, GETDATE());
END
DELIMITER;$$

Answer 1

Thank you guys. I'm a beginner and the tips clarified. I did this:
```lang_sql
DELIMITER $$
CREATE TRIGGER employeeAge
BEFORE INSERT ON employee
FOR EACH ROW
BEGIN
DECLARE dob DATE;
DECLARE ssn_employee1 int;
SELECT new.ssn_employee INTO ssn_employee1;
SELECT person.birth_date into dob FROM person WHERE ssn = ssn_employee1;
SET new.age = TIMESTAMPDIFF(YEAR, dob, NOW());
END;
DELIMITER $$