Interview Questions - DBMS

An ACID transaction is one that is atomic, consistent, isolated, and durable. Durable means that database changes are permanent. Consistency can mean either statement level or transaction level consistency. With transaction level consistency, a transaction may not see its own changes. There are four transaction isolation levels: read committed, read uncommitted, repeatable read and serialization. Atomic means it is performed as a unit.

There are three (3) ways to create an Oracle database. You can create a database using the Database Configuration Assistant, using the Oracle-supplied database creation procedures or using the SQL CREATE DATABASE command.

A sequence is an object that generates a sequential series of unique numbers. Sequences are most often used to provide values for surrogate keys. However, there are three problems with using sequences. First, a developer can use a defined sequence for any purpose; and if a sequence is used for purposes other than the surrogate key, some values will be missing. A second problem is that there is nothing in the schema that prevents someone from issuing an INSERT statement that adds data to the table but that does not use the sequence. Finally, it is possible to use the wrong sequence when putting data into a table.

Indexes can be created to enforce uniqueness, to facilitate sorting, and to enable fast retrieval by column values. A good candidate for an index is a column that is frequently used with equal conditions in WHERE clauses.