This would be an
additional and custom referential integrity constraint.
Another key facility offered by a DBMS to support data integrity is the transaction. A transaction is
a mechanism for grouping related changes to the database for those occasions when it??™s important that either all
changes occur, or that nothing at all changes.
160 DATABASE [CHAP. 8
As we described earlier in this chapter, the familiar example is the act of moving money from a savings
account to a checking account. The customer thinks of the act as a single action, but the database must take two
actions. The database must reduce the balance in the savings account, and increase the balance in the checking
account. If the first action should succeed, and the second fail (perhaps the computer fails at that instant), the
customer will be very unhappy, for their savings account will contain less and their checking account will contain
what it did. The customer would prefer that both actions be successful, but if the second action fails, the
customer wants to be sure that everything will be put back as it was initially. Either every change must be successful,
or no change must occur.
Database management systems allow the user (or programmer) to specify transaction boundaries.
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