Question

I have a table

car

 id |  person_id  |  mpg
------------------------  
 4  |     1       |  50
 5  |     1       |  15
 6  |     2       |  10
 7  |     2       |  28
 8  |     3       |  33

I need to get an average of each person’s mpg and then an average for the group.

person 1 avg = (50 + 15) / 2 = 32.5
person 2 avg = (10 + 28) / 2 = 19
person 3 avg = 33

group average = 32.5 + 19 + 33 / 3 = 28.1

Is there a query that will do what I need?

Answer 1

SELECT person_id, AVG(mpg) from car group by person_id;

If you want to get an average for the group, you should probably do this:

SELECT AVG(mpg) from car;

Unless you really want to average the averages, which seems a bit dubious to me:

SELECT AVG(average) from (SELECT person_id, AVG(mpg) as average from car group by person_id);