SQL Guide : Getting started with SQL : More about transactions
  
More about transactions
As described in the previous section, SQL allows you to group multiple statements into a single “atomic” (indivisible) piece of work called a transaction. Successful transactions are preserved with the command COMMIT WORK. Below is a simplistic example.
COMMIT WORK; -- Finish the previous transaction.
UPDATE stores SET balance = balance + 199.95
  WHERE store_name = 'Big Tyke Bikes';
UPDATE checking_accounts SET balance = balance - 199.95
  WHERE name = 'Jay Smith';
COMMIT WORK;
If you do not want to keep a particular transaction, you can roll it back by using the command:
ROLLBACK WORK;
If you do not explicitly commit or roll back your work, then the server will roll it back for you. In other words, unless you confirm that you want to keep the data (by committing it), the data will be discarded.
See also
Getting started with SQL