Editing ACID (section)

=== Consistency failure ===
Consistency is a very general term, which demands that the data must meet all validation rules. In the previous example, the validation is a requirement that {{math|{{var|A}} + {{var|B}} {{=}} 100}}. All validation rules must be checked to ensure consistency. Assume that a transaction attempts to subtract 10 from {{var|A}} without altering {{var|B}}. Because consistency is checked after each transaction, it is known that {{math|{{var|A}} + {{var|B}} {{=}} 100}} before the transaction begins. If the transaction removes 10 from {{var|A}} successfully, atomicity will be achieved. However, a validation check will show that {{math|{{var|A}} + {{var|B}} {{=}} 90}}, which is inconsistent with the rules of the database. The entire transaction must be canceled and the affected rows rolled back to their pre-transaction state. If there had been other constraints, triggers, or cascades, every single change operation would have been checked in the same way as above before the transaction was committed. Similar issues may arise with other constraints. We may have required the data types of both {{var|A}} and {{var|B}} to be integers. If we were then to enter, say, the value 13.5 for {{var|A}}, the transaction will be canceled, or the system may give rise to an alert in the form of a trigger (if/when the trigger has been written to this effect). Another example would be integrity constraints, which would not allow us to delete a row in one table whose primary key is referred to by at least one [[foreign key]] in other tables.