Transactions 
A database transaction is a sequence of SQL statements that the database system treats as a unit. A transaction brings the database from one consistent state to another. If a transaction is interrupted, then the database system returns the database to the state it was in before the transaction began (ROLLBACK).
Caution
This section deals with database transactions. In SAP systems, the term transaction has another meaning.
Transition of the Database from one Consistent State to Another by Way of Intermediate States
When you log on to a database, the database system opens the first transaction.
A transaction is always completed by a COMMIT or ROLLBACK:
When a transaction is successfully concluded with a COMMIT, all of the data changes are retained.
If a transaction is ended with a ROLLBACK or terminated in any other way, the database system reverses all the data changes made during the transaction.
Both COMMIT and ROLLBACK open a new transaction implicitly.
The transaction manager of the database system ensures, among other things, that parallel transactions from multiple database sessions are processed correctly, and deliver the same results as they would if processed sequentially. Since competing transactions can access the same database objects, the database system uses locks to isolate individual transactions. More information: Locks
You can explicitly open, cancel and exit transactions.
More information:
SQL Reference Manual, Transactions
Database Administration in CCMS, Transactions
All transactions that change data are logged by the database system in log entries.
More information: How Databases Log Data Changes
Transactions for OMS Data (Only for SAP liveCache Technology)
When executing transactions for OMS data, the database system makes use of a consistent view. For each transaction, the database system copies the objects to the OMS heap when it reads them. When the transaction is closed with a COMMIT, the database system copies changes to the objects from the OMS heap to the data cache and then removes all objects of the transaction from the OMS heap.