count total no of people in school

database for school their are two main table student and teacher the creation of table is almost like this

create table students (
    students_id int,
    year int
)


create table Teacher (
    Teacher_id int,
    year int
)

but i what to count total number of people(student + teacher) in each year.

example

students
    students_id     year
             01     2020
             02     2019
             03     2020
             04     2019

Teacher
    Teacher_id      year
            01      2020
            02      2018
            03      2020
            04      2019


answer
    year    total
    2020        4 (2 student + 2 Teacher)
    2019        3 (2 student + 1 Teacher)
    2018        1 (0 student + 1 Teacher)

Answer

Use union all and aggregation:

select year, count(*)
from ((select year from students) union all
      (select year from teachers)
     ) st
group by year;