SQL: Need to get MAX values from one table and divide by the total number of rows in another table -

i have table bunch of scores lessons, on user user basis

------------------------ |uid |lesson_id |score | ------------------------ |1   |0         |20    | |1   |0         |25    | |1   |0         |15    | |1   |0         |40    | |1   |1         |70    | |1   |0         |10    | |1   |1         |20    | |1   |1         |55    | |1   |1         |55    | |1   |0         |5     | |1   |2         |65    | ------------------------ 

i have table of possible lessons can scored:

------------ |lesson_id | ------------ |0         | |1         | |2         | |3         | |4         | |5         | ------------ 

i need calculate maximum score each lesson in second table scores in first table, , take average of on number of lessons in second table:

so, maximum scores scores table (for user 1):

----------------------- |lesson_id |max_score | ----------------------- |0         |40        | |1         |70        | |2         |65        | ----------------------- 

i need sum them: 175 , divide total number of lessons in table 2: 6 should give answer 29.16

any ideas how in single statement?

i can average of max values scores table (for user 1) so:

select avg(max_score) avg_max_score  (     select uid, lesson_id, max(score) max_score cdu_user_progress      , uid =1     group lesson_id  ) m 

select     avg(max_score)    (      select          lesson.lesson_id,          max(isnull(score,0)) max_score              lesson     left join          cdu_user_progress     on          lesson.lesson_id = cdu_user_progress.lesson_id     group          lesson.lesson_id    ) m