Question

Okay I have two tables:
Table 1 looks like this:

id age gender
1   10   M
2   11   F
3   11   F

And Table 2 looks like this (same with different values):

id age gender
1  11   F
2  12   M
3  10   M

Now I want my final output to look like the following:

age count
10   2
11   3
12   1

What is the most efficient way to achieve this?

Answer 1

You want to aggregate the union:

select age, count(*)
from (select id, age, gender from table1 union all
      select id, age, gender from table2
     ) t
group by age