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Difference between revisions of "EclipseLink/UserGuide/JPA/Basic JPA Development/Querying/JPQL"

(DELETE)
(JPQL)
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==SELECT==
 
==SELECT==
 
Select queries can be used to read objects from the database.  Select queries can return a single object or data element, a list of objects or data elements, or an object array of multiple objects and data.
 
Select queries can be used to read objects from the database.  Select queries can return a single object or data element, a list of objects or data elements, or an object array of multiple objects and data.
 +
 +
====Select Query====
 +
<source lang="java">
 +
Query query = em.createQuery("Select e FROM Employee e WHERE e.salary > 100000");
 +
List<Employee> result = query.getResultList();
 +
</source>
  
 
===FROM===
 
===FROM===
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This example demonstrates how to use an update query to give employees a raise. The <tt>WHERE</tt> clause contains the conditional expression.
 
This example demonstrates how to use an update query to give employees a raise. The <tt>WHERE</tt> clause contains the conditional expression.
  
<span id="Example 21-7"></span>
+
====Update Query====
'''''Update Query'''''
+
 
<source lang="java">
 
<source lang="java">
 
Query query = em.createQuery("UPDATE Employee e SET e.salary = 60000 WHERE e.salary = 50000");
 
Query query = em.createQuery("UPDATE Employee e SET e.salary = 60000 WHERE e.salary = 50000");
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This example demonstrates how to use a delete query to remove all employees who are not assigned to a department. The <tt>WHERE</tt> clause contains the conditional expression.
 
This example demonstrates how to use a delete query to remove all employees who are not assigned to a department. The <tt>WHERE</tt> clause contains the conditional expression.
  
<span id="Example 21-8"></span>
+
====Delete Query====
'''''Delete Query'''''
+
 
<source lang="java">
 
<source lang="java">
 
Query query = em.createQuery("DELETE FROM Employee e WHERE e.department IS NULL");
 
Query query = em.createQuery("DELETE FROM Employee e WHERE e.department IS NULL");

Revision as of 13:42, 16 April 2012

EclipseLink JPA

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JPQL

The Java Persistence Query Language (JPQL) is the query language defined by JPA. JPQL is similar to SQL, but operates on objects, attributes and relationships instead of tables and columns. JPQL can be used for reading (SELECT), as well as bulk updates (UPDATE) and deletes (DELETE). JPQL can be used in a NamedQuery (through annotations or XML) or in dynamic queries using the EntityManager createQuery() API.

Elug javaspec icon.gif

For more information, see Chapter 4 "Query Language" in the JPA Specification.

SELECT

Select queries can be used to read objects from the database. Select queries can return a single object or data element, a list of objects or data elements, or an object array of multiple objects and data.

Select Query

Query query = em.createQuery("Select e FROM Employee e WHERE e.salary > 100000");
List<Employee> result = query.getResultList();

FROM

ORDER BY

GROUP BY

HAVING

UNION

WHERE

UPDATE

You can perform bulk update of entities with the UPDATE statement. This statement operates on a single entity type and sets one or more single-valued properties of the entity subject to the condition in the WHERE clause. Update queries provide an equivalent to the SQL UPDATE statement, but with JPQL conditional expressions. Update queries should only be used for bulk updates, regular updates to objects should be made by using the object's set methods within a transaction and committing the changes.

Update queries return the number of modified rows on the database (row count).

This example demonstrates how to use an update query to give employees a raise. The WHERE clause contains the conditional expression.

Update Query

Query query = em.createQuery("UPDATE Employee e SET e.salary = 60000 WHERE e.salary = 50000");
int rowCount = query.executeUpdate();

The persistence context is not updated to reflect results of update operations. If you use a transaction-scoped persistence context, you should either execute the bulk operation in a transaction all by itself, or be the first operation in the transaction. That is because any entity actively managed by the persistence context will remain unaware of the actual changes occurring at the database level.

The objects in the shared cache that match the update query will be invalidated to ensure subsequent persistence contexts see the updated data.

DELETE

You can perform bulk removal of entities with the DELETE statement. Delete queries provide an equivalent to the SQL DELETE statement, but with JPQL conditional expressions. Delete queries should only be used for bulk deletes, regular deletes to objects should be performed through calling the EntityManager remove() API.

Delete queries return the number of deleted rows on the database (row count).

This example demonstrates how to use a delete query to remove all employees who are not assigned to a department. The WHERE clause contains the conditional expression.

Delete Query

Query query = em.createQuery("DELETE FROM Employee e WHERE e.department IS NULL");
int rowCount = query.executeUpdate();

Elug note icon.png

Note: Delete queries are polymorphic: any entity subclass instances that meet the criteria of the delete query will be deleted. However, delete queries do not honor cascade rules: no entities other than the type referenced in the query and its subclasses will be removed, even if the entity has relationships to other entities with cascade removes enabled. Delete queries will delete the rows from join and collection tables.


The persistence context is not updated to reflect results of delete operations. If you use a transaction-scoped persistence context, you should either execute the bulk operation in a transaction all by itself, or be the first operation in the transaction. That is because any entity actively managed by the persistence context will remain unaware of the actual changes occurring at the database level.

The objects in the shared cache that match the delete query will be invalidated to ensure subsequent persistence contexts do not see the removed objects.

Functions

JPQL supports several database functions. These functions are database independent in name and syntax, but require database support. If the database supports an equivalent function or different syntax the standard JPQL function is supported, if the database does not provide any way to perform the function, then it is not supported. EclipseLink provides support for several functions beyond the JPA spec. EclipseLink also supports calling specific database functions through FUNCTION, FUNC, and OPERATOR.

JPQL supported functions:

  • + (addition)
  • - (subtraction)
  • / (division)
  • * (multiplication)
  • ABS
  • CASE
  • COALESCE
  • CONCAT
  • CURRENT_DATE
  • CURRENT_TIME
  • CURRENT_TIMESTAMP
  • LENGTH
  • LOCATE
  • LOWER
  • MOD
  • NULLIF
  • SQRT
  • SUBSTRING
  • TRIM
  • UPPER

EclipseLink support functions:

  • CAST
  • EXTRACT
  • REGEXP

INDEX, KEY, SIZE, IS EMPTY, TYPE

EclipseLink Extensions (EQL)

EclipseLink provides many extensions to the standard JPA JPQL. These extensions provide access to additional database features many of which are part of the SQL standard, provide access to native database features and functions, and provide access to EclipseLink specific features. EclipseLink JPQL extensions are referred to as the EclipseLink Query Language (EQL).

EclipseLink's JPQL extensions include:

  • Less restrictions than JPQL, allows sub-selects and functions within operations such as LIKE, IN, ORDER BY, constructors, functions etc.
  • Allow != in place of <>
  • FUNC operation to call database specific functions (now FUNCTION in JPA 2.1)
  • TREAT operation to downcast related entities with inheritance
  • OPERATOR operation to call EclipseLink database independent functions (EL 2.4)
  • SQL operation to mix SQL with JPQL (EL 2.4)
  • CAST and EXTRACT functions (EL 2.4)
  • REGEXP function for regular expression querying (EL 2.4)
  • Usage of sub-selects in the SELECT and FROM clause (EL 2.4)
  • ON clause support for defining JOIN and LEFT JOIN conditions (EL 2.4)
  • Joins between independent entities (EL 2.4)
  • Usage of an alias on a JOIN FETCH (EL 2.4)
  • COLUMN operation to allow querying on non mapped columns (EL 2.4)
  • UNION, INTERSECT, EXCEPT support (EL 2.4)
  • Usage of object variables in =, <>, IN, IS NULL, and ORDER BY


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