MySQL 8.0

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CREATE TABLE user
    (`id` int, `name` varchar(3), `join_date` varchar(10))
INSERT INTO user
    (`id`, `name`, `join_date`)
VALUES
    (1, 'Sam', '25-11-2017'),
    (2, 'Moe', '03-04-2017'),
    (3, 'Tim', '04-07-2018'),
    (4, 'Sal', '30-01-2017'),
    (5, 'Joe', '13-08-2018')
CREATE TABLE allowed_exp_range
    (`starting_exp_months` int, `end_exp_months` int)
INSERT INTO allowed_exp_range
    (`starting_exp_months`, `end_exp_months`)
VALUES
    (0, 6),
    (9, 24)
SELECT id, name
     , (YEAR(CURDATE())*12+MONTH(CURDATE())) - (YEAR(STR_TO_DATE(join_date, '%d-%m-%Y'))*12+MONTH(STR_TO_DATE(join_date, '%d-%m-%Y'))) - 1 AS months
     , DAY(LAST_DAY(STR_TO_DATE(join_date, '%d-%m-%Y')))-DAY(STR_TO_DATE(join_date, '%d-%m-%Y'))+1 AS month1days
     , DAY(CURDATE()) AS month2days
FROM user


id
name
months
month1days
month2days


1
Sam
12
6
11

2
Moe
19
28
11

3
Tim
4
28
11

4
Sal
22
2
11

5
Joe
3
19
11


SELECT id
     , name
     , (YEAR(CURDATE())*12+MONTH(CURDATE())) - (YEAR(STR_TO_DATE(join_date, '%d-%m-%Y'))*12+MONTH(STR_TO_DATE(join_date, '%d-%m-%Y'))) - 1 -- whole months
       + CASE WHEN DAY(LAST_DAY(STR_TO_DATE(join_date, '%d-%m-%Y'))) - DAY(STR_TO_DATE(join_date, '%d-%m-%Y')) + 1 + DAY(CURDATE()) > 15 THEN 1 ELSE 0 END -- broken month
       AS months
FROM user


id
name
months


1
Sam
13

2
Moe
20

3
Tim
5

4
Sal
22

5
Joe
4


SELECT u.id
     , u.name
     , a.starting_exp_months
     , a.end_exp_months
FROM user u
JOIN allowed_exp_range a
ON (YEAR(CURDATE())*12+MONTH(CURDATE())) - (YEAR(STR_TO_DATE(u.join_date, '%d-%m-%Y'))*12+MONTH(STR_TO_DATE(u.join_date, '%d-%m-%Y'))) - 1
       + CASE WHEN DAY(LAST_DAY(STR_TO_DATE(u.join_date, '%d-%m-%Y'))) - DAY(STR_TO_DATE(u.join_date, '%d-%m-%Y')) + 1 + DAY(CURDATE()) > 15 THEN 1 ELSE 0 END
       BETWEEN a.starting_exp_months AND a.end_exp_months


id
name
starting_exp_months
end_exp_months


1
Sam
9
24

2
Moe
9
24

3
Tim
0
6

4
Sal
9
24

5
Joe
0
6