mySQL Populate DB Column using calculated value from anouther table - mysql

Question

What I am trying to do is have a field called 'age' autopopulate from a persons date of birth when a row is added - the trick is the persons date of birth resides in anouther field.
My two tables are:
student
student_id (PK), first_name, last_name, date_of_birth
fitness_report
report_id (PK), test_date, test_period, age_tested, student_id (FK)
ideally the age_tested will be caluclated from the test_period however happy to use ()NOW as that'll be within reasonable limits.
Obviously what i need to do here is create a trigger - but not sure on the SELECT statement to get the age to populate. Help is much appreciated.

Answer 1

As rwilliams said, it is not advisable to store the age_tested as it is redundant but if it is what you want to do you can wrap a simple trigger around Mosty's solution -
CREATE TRIGGER fitness_report_insert BEFORE INSERT ON `fitness_report`
FOR EACH ROW
BEGIN
SET NEW.age_tested = (SELECT DATE_FORMAT(NEW.test_period, '%Y') - DATE_FORMAT(student.date_of_birth, '%Y') - (DATE_FORMAT(NEW.test_period, '00-%m-%d') < DATE_FORMAT(student.date_of_birth, '00-%m-%d')) FROM student WHERE student.student_id = NEW.student_id);
END
This assumes that test_period is a date as previously mentioned by Mosty. I have not tried this as I do not have access to a server right now so it may need a little tweaking.
UPDATE: You could try the following as a starting point for your stats -
SELECT
DATE_FORMAT(test_period, '%Y') - DATE_FORMAT(date_of_birth, '%Y') - (DATE_FORMAT(test_period, '00-%m-%d') < DATE_FORMAT(date_of_birth, '00-%m-%d')) AS age,
s.gender,
AVG(r.score) AS score
FROM student s
INNER JOIN fitness_report r
ON s.student_id = r.student_id
GROUP BY age, s.gender

Answer 2

This is how to get current age:
select s.name, s.date_of_birth,
date_format(now(), '%Y') - date_format(date_of_birth, '%Y') -
(date_format(now(), '00-%m-%d') < date_format(date_of_birth, '00-%m-%d'))
as age
from s
Example
This is how to get age up to the test_period (shouldn't it be test_date?):
select s.name, s.date_of_birth, r.test_period,
date_format(test_period, '%Y') - date_format(date_of_birth, '%Y') -
(date_format(test_period, '00-%m-%d') < date_format(date_of_birth, '00-%m-%d'))
as age
from r
join s on s.id_student = r.student_id
Example