Understanding EclipseLink,
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In general, querying a data source means performing an action on or interacting with the contents of the data source. To do this, you must be able to perform the following:
Define an action in a syntax native to the data source being queried.
Apply the action in a controlled fashion.
Manage the results returned by the action (if any).
You must also consider how the query affects the EclipseLink cache.
This section introduces query concepts unique to EclipseLink, including the following:
The Call object encapsulates an operation or action on a data source. The EclipseLink API provides a variety of Call types such as structured query language (SQL), Java Persistence Query Language (JPQL), and Extensible Markup Language (XML).
You can execute a Call directly or in the context of a DatabaseQuery.
A DatabaseQuery object is an abstraction that associates additional customization and optimization options with the action encapsulated by a Call. By separating these options from the Call, EclipseLink can provide sophisticated query capabilities across all Call types.
Queries can be defined for objects or data, as follows:
Object-level queries are object-specific and return data as objects in your domain model. They are the preferred type of query for mapped data. By far, object-level DatabaseQuery queries are the most common query used in EclipseLink.
Data-level queries are used to query database tables directly, and are an appropriate way to work with unmapped data.
While data-level queries return raw data and object-level queries return objects in your domain model, summary queries return data about objects. EclipseLink provides partial object queries to return a set of objects with only specific attributes populated, and report queries to return summarized (or rolled-up) data for specific attributes of a set of objects.
In addition to storing named queries applicable to a particular class, you can also use the DescriptorQueryManager to override the default action that EclipseLink defines for common data source operations.
EclipseLink expressions let you specify query search criteria based on your domain object model. When you execute the query, EclipseLink translates these search criteria into the appropriate query language for your platform.
The EclipseLink API provides the following two public classes to support expressions:
The Expression class represents an expression that can be anything from a simple constant to a complex clause with boolean logic. You can manipulate, group, and integrate expressions.
The ExpressionBuilder class is the factory for constructing new expressions.
You can specify a selection criterion as an Expression with DatabaseQuery method setSelectionCriteria, and in a finder that takes an Expression.
For more information about using EclipseLink expressions, see Chapter 11, "Understanding EclipseLink Expressions".
A query key is a schema-independent alias for a database field name. Using a query key, you can refer to a field using a schema-independent alias. In relational projects only, EclipseLink automatically creates query keys for all mapped attributes. The name of the query key is the name of the class attribute specified in your object model.
You can configure query keys in a class descriptor or interface descriptor. You can use query keys in expressions and to query variable one-to-one mappings.
By default, EclipseLink creates query keys for all mapped attributes, but in some scenarios you may find it beneficial to add your own.
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