SELECT

  AVG(ABS(CAST(r.rev_len AS SIGNED) - CAST(p.rev_len AS SIGNED))) AS avg_edit_size

FROM (revision r

JOIN revision p ON r.rev_parent_id = p.rev_id

JOIN (

  SELECT rev_actor, COUNT(*) AS edit_count

  FROM revision     # number of edits done by user

  GROUP BY rev_actor

) AS user_edits ON r.rev_actor = user_edits.rev_actor)

where user_edits.edit_count between 11 and 50    # we can change range of edits 

​

;