title: How to find unmatched records in a single table?
body: <p>I'm scraping a log file for transaction records that I am inserting into a table that will be used for several mining tasks. Each record has (among other things) an ID and a transaction type, either request or response. A request/response pair will have the same ID.</p>
  
  <p>One of my tasks is to find all of the requests that do not have a corresponding response. I thought about joining the table to itself, where A.ID = B.ID AND A.type = 'req' and B.type = 'res', but that gives me the opposite of what I need.</p>
  
  <p>Since the IDs will always occur either once or twice, is there a query that would select ID where there is only one occurrence of that ID in the table? </p>