sql - SQL Selecting average of rating from another table

Question

I have two tables: Anime_List and Anime_Reviews.

Anime_List table has a 'Rating' field that grabs the average rating from Anime_Reviews. However, some ratings (rows) don't exist in Anime_Reviews and so my current query only returns titles that have a average rating.

SELECT Anime_List.AnimeID, Anime_List.Name, Anime_List.AnimeImage, Anime_List.Synopsis, Anime_List.Type, Anime_List.Episodes, Anime_List.Genres, AVG(Anime_Reviews.Rating) 
FROM Anime_List INNER JOIN Anime_Reviews ON Anime_List.AnimeID = Anime_Reviews.AnimeID
GROUP BY Anime_List.AnimeID, Anime_List.Name, Anime_List.AnimeImage, Anime_List.Synopsis, Anime_List.Type, Anime_List.Episodes, Anime_List.Genres

How do I return the value of 0.0 if there are no ratings (rows) in Anime_Reviews for each title listed in Anime_List?

Answer 1

A version using a subquery

SELECT l.AnimeID, 
       l.Name, 
       l.AnimeImage, 
       l.Synopsis, 
       l.Type, 
       l.Episodes, 
       l.Genres, 
       (SELECT ISNULL(AVG(Rating), 0) Rating
          FROM Anime_Reviews 
         WHERE AnimeID = l.AnimeID)
  FROM Anime_List l

Answer 2

Use LEFT JOIN with COALESCE:

SELECT 
  l.AnimeID, 
  l.Name, 
  l.AnimeImage, 
  l.Synopsis, 
  l.Type, 
  l.Episodes, 
  l.Genres, 
  COALESCE(AVG(r.Rating), 0) AS Rating 
FROM Anime_List AS l
LEFT JOIN Anime_Reviews AS r ON l.AnimeID = r.AnimeID
GROUP BY l.AnimeID, 
         l.Name, 
         l.AnimeImage, 
         l.Synopsis, 
         l.Type, 
         l.Episodes, 
         l.Genres;

For more information about the different types of SQL JOIN see this article please:

• A Visual Explanation of SQL Joins.