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Databases Reporting with SQL Aggregate and Numeric Functions Practice Session

Jonatan Spahn
Jonatan Spahn
6,362 Points
Posted by Jonatan Spahn
Jonatan Spahn
6,362 Points

Average of each movie

The third practice problem in the SQL playground asks you to calculate the average rating for each movie and round it.

I see that the following code has been posted from another asking a question about this. The code was as follows: SELECT ROUND(AVG(rating,1) AS average_rating FROM reviews

How would you get the AVG for EACH movie though? I tried starting simple with the following: SELECT DISTINCT movie_id FROM reviews; which return each movie_id which were 6,2, and 3, then I tried: SELECT DISTINCT movie_id, AVG(rating) FROM reviews; but that brought it up to two columns one with the count of distinct movie_id's which is 3 and one column with the average rating.

but how do you keep each distinct id and average each rating for each movie.

Thank you for any help you can offer!

Antonio De Rose
Antonio De Rose
20,884 Points
Antonio De Rose
Antonio De Rose
20,884 Points 
SELECT ROUND(AVG(rating,1)) AS average_rating_movie, movie_id 
FROM reviews
GROUP BY movie_id

3 Answers

Jay Meyer
Jay Meyer
7,708 Points
Jay Meyer
Jay Meyer
7,708 Points

Hey Jonatan, I'm not 100% sure I know what your asking, but I used:

SELECT movie_id, ROUND(AVG(rating),1) AS average_rating FROM reviews GROUP BY movie_id;

This brings up each movies ID and their corresponding AVG review. So it lists rows for id's 2, 3, and 6 and the average rating for each of those ID's. Hope this is what you were struggling with.

Dennis Amiel Domingo
Dennis Amiel Domingo
15,357 Points
Dennis Amiel Domingo
Dennis Amiel Domingo
15,357 Points

The above answers are correct!

Though I went further since I wasn't satisfied with just looking at the ID's of the movies. I wanted to see the names of the movie beside the ratings.

So this is what I did.

SELECT title, movie_id, ROUND(AVG(rating),1) AS average_rating 
FROM reviews, movies 
WHERE reviews.movie_id = movies.id 
GROUP BY movie_id;

You can also remove the movie_id in SELECT if you like.

Ethan Martin
Ethan Martin
Courses Plus Student 1,883 Points
Ethan Martin
Ethan Martin
Courses Plus Student 1,883 Points

How does the code know which columns apply to each table? It doesn't appear the order of the tables after FROM matters or even the order in the WHERE clause.

Dennis Amiel Domingo
Dennis Amiel Domingo
15,357 Points
Dennis Amiel Domingo
Dennis Amiel Domingo
15,357 Points

Ethan Martin, you need to include the name of the table to the column name, like this:

SELECT <table 1>.<column name>, <table 2>.<column name> FROM <table 1>, <table 2>;

SELECT reviews.title, movies.title FROM reviews, movies;