Show AVG evaluation in SQL

Question

I started SQL a few weeks ago so I'm fairly new at this. I encountered this exercise in my homework and i was a bit confused or unsure if i did them correctly? Im posting this here to check if i did any mistakes or not. If possible can you tell me if i did them correctly and in case i didnt maybe point out the mistakes and maybe provide some hints on how to fix them?

So this is the exercise

1. Given the following relational schema:

USER (SSN, NameU, SurnameU, City, YearOfBirth, UserType)

MOVIE (CodM, Title, Nation, Language, MovieStudio, Genre)

EVALUATION (SSN, CodM, Evaluation, Date)

a. For each user type, show the average evaluation given to movies produced by "Marvel" (MovieStudio = "Marvel").

SELECT  AVG(EVALUATION)

FROM    EVAUATION E, USER U, MOVIE M 

WHERE   E.SSN=U.SSN AND E.CODM=M.CODM AND MOVIESTUDIO='MARVEL'  

For the first part i'm a bit confused in the part where it says for each user type . I dont think it is correct but how do i show information about each user type

Answer 1

First, nearly 30 years ago the ANSI-92 standard was adopted to replace using , for implicit joins between tables, instead using explicit JOIN syntax. I strongly recommend avoiding any and all material that attempts to teach you that using , is correct; it's not, use JOIN syntax, it's already close to thirty years old...

Finally, you need to look at using GROUP BY as part of your aggregation, to get multiple results rather than just one...

SELECT
  U.UserType, AVG(EVALUATION)
FROM
  USER U
INNER JOIN
  EVAUATION E
    ON E.SSN=U.SSN
INNER JOIN
  MOVIE M 
    ON E.CODM=M.CODM
WHERE
  M.MOVIESTUDIO='MARVEL'
GROUP BY
  U.UserType

This will give one row per UserType .