Average cost of purchase orders made during the first six months of 2012.
purchase orders refers to TransactionType = 'P'; first six months of 2012 refers to TransactionDate bewteen '2012-01-01'and '2012-06-30'; average = DIVIDE(ActualCost where TransactionType = 'P', count(TransactionID))

SELECT CAST(SUM(ActualCost) AS REAL) / COUNT(TransactionID) FROM TransactionHistoryArchive WHERE TransactionType = 'P' AND TransactionDate >= '2012-01-01' AND TransactionDate < '2012-07-01'