Average over results from query

[KeesSmit]

I would like to write a query in which i calculate the average over the result from an earlier query.
I have a transaction header table and a separate transaction lines table. I am trying to count an amount per transaction by summing the lines table, grouped by transactionnumber and would like to calculate the average over all of these sums.
Any suggestion?
Im using a Mysql that doesn't support VIEWs

 

[r937]

SELECT AVG(amt_per_trans) AS avg_amt_per_trans
  FROM (
       SELECT TH.transactionnumber
            , SUM(TL.amount) AS amt_per_trans
         FROM transaction_headers as TH
       INNER
         JOIN transaction_lines AS TL 
           ON TL.transactionnumber = TH.transactionnumber
       GROUP
           BY TH.transactionnumber
       ) AS trans_totals

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