SQL分组平均值

Question

Hi! I'm new to SQL I'm having trouble correctly using the avg and sum function. I'm working with the database above. I need to find the average profit for each movie star but only output that average only if the sum of their profits is > 200.

 SELECT MovieStar.sname, avg(profit) From MovieStar, Movie
 GROUP BY sname
 HAVING sum(Movie.profit) > 200

I'm using SQL Fiddle to try and figure this out, but it seems to be returning the avg of the WHOLE profit column and not each actor but I'm not sure what I could be doing wrong. How can I approach this so I can get the avg of each actor and not the avg of the whole profit column? Here is the SQL Fiddle I made. Thanks in advance!

Answer 1

Looks like the query is doing a CROSS JOIN operation, a cartesian product, matching every movie to each moviestar. Seems like we would only want to match a moviestar to particular movies, probably using (including) the starsin table to get the matches.

I recommend you ditch the old-school comma operator the join operation. Use the newer JOIN keyword instead. And put the join predicates in the appropriate ON clause instead of the WHERE clause.

Also, best practice is to qualify all column references; even when not required to eliminate ambiguity, it prevents the query from breaking when new columns are added, and it aids the future reader... the poor soul who has to go look at the table definitions to figure out which columms come from which tables.

I think you want something like this:

 SELECT ms.sname
 --   , ms.sno
      , AVG(m.profit)  AS  avg_profit
 --   , SUM(m.profit)  AS  tot_profit 
   FROM MovieStar ms
   JOIN StarsIn si 
      ON si.sno = ms.sno
    JOIN Movie m
      ON m.mno = si.mno
   GROUP BY ms.sno, ms.sname
  HAVING SUM(m.profit) > 200
   ORDER BY AVG(m.profit) DESC