Subqueries, or nested queries, are used to bring back a set of rows to be used by the parent query. Depending on how the subquery is written, it can be executed once for the parent query or it can be executed once for each row returned by the parent query. If the subquery is executed for each row of the parent, this is called a correlated subquery.

A correlated subquery can be easily identified if it contains any references to the parent subquery columns in its WHERE clause. Columns from the subquery cannot be referenced anywhere else in the parent query. The following example demonstrates a non-correlated subquery.

It is a program module that provides the interface between the low-level data stored in database, application programs and queries submitted to the system.

Defines the queries for the finder and select methods of an entity bean having container-managed persistence. A subset of

SQL92, EJB QL has extensions that allow navigation over the relationships defined in an entity bean's abstract schema.

A logical name that is referenced in EJB QL queries.

Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table. Hide data complexity. Simplify commands for the user. Present the data in a different perspectives from that of the base table. Store complex queries.

Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.