SQL - average distance per day

Question

I have a problem here: "Show average distance per day driven by cars from Paris"

I have also 2 tables referred to this problem

1. table_cars : id , brand , type , license
2. table_distances : id_car , date , distance

I have managed to select "the average distance for the cars from Paris"

 select avg(table_distances.distance)
     from table_distances
     INNER JOIN table_cars ON table_distances.id_car = table_cars.id
     where table_cars.license = 'Paris';'

Though, I have still a problem with average distance per day. I looked over related questions on the stackoverflow/google but I got more confused.

Can somebody explain how I can improve my query to show average distance per day?

Answer 1

Simply add the date to what you select and group by it so it's averaged per row:

SELECT table_distances.date, avg(table_distances.distance)
FROM table_distances
  INNER JOIN table_cars ON table_distances.id_car = table_cars.id
WHERE table_cars.license = 'Paris'
GROUP BY table_distances.date

Answer 2

This should get you the distances per car per date.

SELECT id_car, date, AVG(table_distances.distance) 
FROM table_distances
INNER JOIN table_cars 
ON table_distances.id_car = table_cars.id
WHERE table_cars.license = 'Paris'
GROUP BY id_car, date
ORDER BY id_car, date

Answer 3

Simply add GROUP BY clause to your DATE column

Reference for SQL GROUP BY clause

SELECT AVG(td.distance)
     FROM table_distances td
     INNER JOIN table_cars tc ON td.id_car = tc.id
     WHERE tc.license = 'Paris'
     GROUP BY td.date;

Then you have to get average distance of a car per day