Transaction Definition in SQL

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Transaction Definition in SQL

A data-manipulation language must include a construct for specifying the set of actions that constitute a transaction.

The SQL standard specifies that a transaction begins implicitly. Transactions are ended by one of these SQL statements:

• Commit work commits the current transaction and begins a new one.

• Rollback work causes the current transaction to abort.

The keyword work is optional in both the statements. If a program terminates with- out either of these commands, the updates are either committed or rolled back — which of the two happens is not specified by the standard and depends on the implementation.

The standard also specifies that the system must ensure both serializability and freedom from cascading rollback. The definition of serializability used by the standard is that a schedule must have the same effect as would some serial schedule. Thus, conflict and view serializability are both acceptable.

The SQL-92 standard also allows a transaction to specify that it may be executed in a manner that causes it to become nonserializable with respect to other transactions.

We study such weaker levels of consistency in Section 16.8.

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