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find top 3 values with an inner join
username  |  price  |   zip   |  program  |  active
-----------+---------+---------+-----------+---------
 joe       |    5    |  92108  |  dog      |    1
 tom       |    7    |  92108  |  dog      |    1
 mary      |    5    |  92108  |  dog      |    1
 paul      |    6    |  92108  |  dog      |    1
 ron       |    6    |  92108  |  dog      |    1

username  |  balance
-----------+----------
 joe       |    10
 tom       |    12
 mary      |    2
 paul      |    14
 ron       |    3

SELECT SUM(price) AS SumOfTopValues
FROM (
    SELECT users_preferred_zips . * , users.last_purchase, users.lesson_type, users.pref_acct_balance
    INNER JOIN users ON ( users_preferred_zips.username = users.username )
    WHERE users_preferred_zips.zip =  '92108'
    AND users_preferred_zips.program =  'dog'
    AND users_preferred_zips.active =  1
    AND users.pref_acct_balance >= '5'
    ORDER BY price DESC
    LIMIT 3
) AS sub

joe   |  5
tom   |  7
paul  |  6

SELECT SUM(price) AS SumOfTopValues
FROM users_preferred_zips
WHERE username IN (
  SELECT username
  FROM users
  WHERE pref_acct_balance >= 5
)