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Oracle Locks
Oracle Tips by Burleson
 

Oracle uses a sophisticated lock mechanism to prevent multiple users from altering the same data at the same time. This mechanism is typically invisible to database users. Itís not uncommon, especially during system development or after system hang-ups, for a deadlock to exist.

Consider this situation: user jallen has the STUDENTS table locked and needs to obtain a lock on the STUDENT_FINANCIAL_AID table. User msmith has the STUDENT_FINANCIAL_AID table locked and needs to obtain a lock on the STUDENTS table. This is a deadlock, because each user is preventing the other from completing a transaction.

Locks are cleared when a user issues a COMMIT or ROLLBACK statement. Locks can also be explicitly obtained, but this is a rare event. Itís best to let Oracle determine which objects that you need to lock.

NULL Values

A NULL value is a column that does not have a defined value. A NULL value is never equal to any other value, including zero and NULL. The expression

NULL = NULL

returns a FALSE result. Any column in a table that is not constrained by a NOT NULL or primary key constraint can contain a NULL value.

You can test for NULL values in a column using the IS NULL operator:

WHERE <column> IS NULL
WHERE <column> IS NOT NULL

In reality, unexpected NULL values can cause all sorts of heartache for developers. The best way to handle this situation is to be aware that columns can contain NULL values while youíre coding and to be on the lookout for situations in which NULL values are likely to be present.

 

This is an excerpt from the book "High Performance Oracle Database Automation" by Jonathan Ingram and Donald K. Burleson, Series Editor.

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