Editing Consistency (database systems) (section)

{{Short description|Guarantee of validity following an update}}
{{About||Consistency in distributed systems as defined in the CAP Theorem|CAP theorem}}

In [[database systems]], '''consistency''' (or '''correctness''') refers to the requirement that any given [[database transaction]] must change affected data only in allowed ways. Any data written to the database must be valid according to all defined rules, including [[Integrity constraints|constraints]], [[Cascading rollback|cascades]], [[Database trigger|triggers]], and any combination thereof.  This does not guarantee correctness of the transaction in all ways the application programmer might have wanted (that is the responsibility of application-level code) but merely that any programming errors cannot result in the violation of any defined database constraints.<ref name="Date2012">C. J. Date, "SQL and Relational Theory: How to Write Accurate SQL Code 2nd edition", ''O'reilly Media, Inc.'', 2012, pg. 180.</ref>

In a distributed system, referencing [[CAP theorem]], consistency can also be understood as after a successful write, update or delete of a Record, any read request immediately receives the latest value of the Record.