By default Sybase ASE uses 'UNCHAINED' transactional mode. What this means is that any data modification command (ie. UPDATE, DELETE, INSERT) is a transaction and will implicitly generate a BEGIN TRAN/COMMIT TRAN.
This also means, that a normal data modification command (ie. a DELETE), cannot be reverted once it has been executed.
If you want to change this behaviour, you'll have to do one of the following:
* Use the SET CHAINED ON command (OFF is the default) -- this will implicitly start a new transaction with the first data modification command you issue. You'll have to explicitly end your transaction with a COMMIT/ROLLBACK command.
* Use BEGIN TRAN and COMMIT/ROLLBACK to define where your transaction begins / ends.