Show the minimum amount of transactions whose type code is "PUR" and whose share count is bigger than 50.
return transactions ;return share counts of  #1 ;return #1 where #2 is  more than 50 ;return #3 whose type code is  PUR ;return amounts of  #4 ;return the  minimum of  #5
['select', 'project', 'comparative', 'filter', 'project', 'aggregate']