>>In order to use SQLCOMMIT/SQLROLLBACK, you need to issue a SQLSETPROP for the connection immediately before the start of the changes being submited as a transaction, telling the backend to operate in Mnual Transaction mode. You can then submit SQLEXECs to you heart's content. Once you've finished, if all SQLEXECs completed successfully and you want the changes made committed, you issue a SQLCOMMIT, otherwise, a SQLROLLBACK. In order to prevent the next set of SQLEXECs from being treated as a manual transaction, you use SQLSETPROP to reset the transaction processing.
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>What would happen if the system hangs before it gets to SQLCOMMIT/SQLROLLBACK?

If the session disconnects before a transaction completes, the transaction will be rolled back.