Editing ACID (section)

=== Atomicity ===
{{Main article|Atomicity (database systems)}}
[[Database transaction|Transactions]] are often composed of multiple [[SQL syntax|statements]]. [[Atomicity (database systems)|Atomicity]] guarantees that each transaction is treated as a single "unit", which either succeeds completely or fails completely: if any of the statements constituting a transaction fails to complete, the entire transaction fails and the database is left unchanged. An atomic system must guarantee atomicity in each and every situation, including power failures, errors, and crashes.<ref>{{cite web
| access-date = 2011-03-23
| website = webopedia.com
| publisher = Webopedia
| title = Atomic operation
| date = 25 November 2003
| quote = An operation during which a processor can simultaneously read a location and write it in the same bus operation. This prevents any other processor or I/O device from writing or reading memory until the operation is complete.
| url = http://www.webopedia.com/TERM/A/atomic_operation.html}}</ref> A guarantee of atomicity prevents updates to the database from occurring only partially, which can cause greater problems than rejecting the whole series outright. As a consequence, the transaction cannot be observed to be in progress by another database client. At one moment in time, it has not yet happened, and at the next, it has already occurred in whole (or nothing happened if the transaction was cancelled in progress).