Summing across columns

First of all, congratulations on understanding the difference between NULL and zero.

The best way to do this, of course, is to normalize your table. Create a second table, give it a primary key of playerid and weekno, with one column for the score for that week. If a player did not play a particular week, there would be no row for that player that week. This structure is better in a number of ways, not least of which is that you can then use SUM().

With your structure, there's no alternative but to do the addition across each row. Here, you have to be careful not to add in a NULL, because then the row summary will be NULL. Therefore, you have to use COALESCE.

select playerno
     , coalesce(week1,0) + 
       coalesce(week2,0) + 
       coalesce(week3,0) + 
       coalesce(week4,0) + ...
         as playersum
  from scores
group
    by playerno

This is fine, as far as it goes, but you will have even more of a headache if you have to find the average score for the games that the player did play, because then you have to count the number of scores that aren't null.

select playerno
     , coalesce(week1,0) + 
       coalesce(week2,0) + 
       coalesce(week3,0) + 
       coalesce(week4,0) + ...
         as playersum
     , case when week1 is null
            then 0 else 1 end + 
       case when week2 is null
            then 0 else 1 end + 
       case when week3 is null
            then 0 else 1 end + 
       case when week4 is null
            then 0 else 1 end + ...
         as weeksplayed
     , ( coalesce(week1,0) + 
         coalesce(week2,0) + 
         coalesce(week3,0) + 
         coalesce(week4,0) + ... )
       /
       ( case when week1 is null
              then 0 else 1 end + 
         case when week2 is null
              then 0 else 1 end + 
         case when week3 is null
              then 0 else 1 end + 
         case when week4 is null
              then 0 else 1 end + ... )
           as averagescore
  from scores
 where not ( week1 is null
         and week2 is null
         and week3 is null
         and week4 is null ...)
group
    by playerno

Ewww. Anyhow, imagine you have these queries written and tucked away inside your application, all ready to roll out into production, and somebody comes along and says "Hey, let's extend the season by one more week..."

Oh, and regarding your top 10 question? See FIRST N rows, TOP N rows, LAST N rows, BOTTOM N rows... and imagine substituting the average score calculation above into the variable "foo" in some of those syntax examples.