matlos matlos - 3 years ago 108
SQL Question

SQL select for average from another table

I spent a lot of time building this select, but Im not able to solve it. I have 2 tables. First table is called car and has PK (primary key) id_car and another columns name and so on. Second table is called rating and has colums id_rating (PK), id_car (FK) and rating_value (integer). As you suspect, one car can have more than one ranting. I want to select all cars and I want to know average rating to each car. Finally, I want to order the result by this average desc. I was trying things like this:

SELECT id_car, name, average
FROM car C, rating R
WHERE C.id_car = R.id_car
ORDER BY (average) (
SELECT AVG(rating_value) AS average
FROM rating R
WHERE C.id_car = R.id_car)

but it doesn't work.

Kaf Kaf
Answer Source

For SQL Server; Also I suggest you to use JOIN instead of WHERE table1, table2..

SELECT C.id_car, name, AVG(rating_value) AS average
FROM car C JOIN rating R 
      ON C.id_car = R.id_car
GROUP By C.id_car, name
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