Transaction concept

A transaction is a sequence of related statements which transfer a database from one consistent state to another consistent state.

Transactions are either executed completely or not at all.

Example

One thousand euros are to be transferred from account 1789 to account 1564.

This requires the following steps:

• • • Determine the current balance in account 1789.

    • Check whether the balance in account 1789 is greater than 1,000 euros.

    • Adjust the balance in accounts 1789 and 1564.

The transaction has the following structure:

Start of transaction               --
   Search account 1789              |
   Check whether balance > 1000     |
   ...                               > Transaction bracket
   Adjust balance in account 1789   |
   Adjust balance in account 1564   |
End of the transaction             --