Simplified Syntax - The J2EE 1.4 Tutorial

Simplified Syntax

This section briefly describes the syntax of EJB QL so that you can quickly move on to the next section, Example Queries. When you are ready to learn about the syntax in more detail, see the section Full Syntax.

An EJB QL query has four clauses: SELECT, FROM, WHERE, and ORDER BY. The SELECT and FROM clauses are required, but the WHERE and ORDER BY clauses are optional. Here is the high-level BNF syntax of an EJB QL query:

EJB QL ::= select_clause from_clause 
[where_clause][orderby_clause] 

The SELECT clause defines the types of the objects or values returned by the query. A return type is either a local interface, a remote interface, or a persistent field.

The FROM clause defines the scope of the query by declaring one or more identification variables, which can be referenced in the SELECT and WHERE clauses. An identification variable represents one of the following elements:

The abstract schema name of an entity bean
A member of a collection that is the multiple side of a one-to-many relationship

The WHERE clause is a conditional expression that restricts the objects or values retrieved by the query. Although it is optional, most queries have a WHERE clause.

The ORDER BY clause sorts the objects or values returned by the query into a specified order.

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