15 jpaql

Professional Open Source™

07/17/04

JPA QL

Query API

The EntityManager interface methods for creating query instances, the Query interface methods that define and execute the query, and JPQL are referred to as the query API.

Defining named queries

You must create a named (or static) query before you can use it. It is defined either in the entity using annotations, or in the XML file defining O/R mapping metadata.

Creating a query instance

The EntityManager interface provides several methods to create queries using either JPQL or native SQL statements.

Creating a named query instance

Creating a dynamic query instance

Setting parameters for a query

Setting Named parameters

Setting Positional Parameters

Retrieving a single entity

If your query retrieves multiple instances of Category entities with the same name, it will throw NonUniqueResultException. The persistence provider will throw NoResultException when your query does notretrieve any result.

These exceptions will not roll back the active transactions.

Retrieving an entity using getSingleResult does not require an active transaction.However, if no transactions are available, the retrieved entity will be detached after retrieval.

Retrieving a collection of entities

If getResultList does not retrieve any results for a query, it returns an empty list. No exceptions are thrown.

Retrieving a collection does not require anactive transaction, and if one isn’t available, the retrieved entities will be detached after retrieval.

Paginating through a result list

Specifying query hints

A query hint is a tip that is used by the persistence provider while executing queries or retrieving entities.

For example, a hint can be a directive to the persistence provider whether to use a cache while executing a query.

Specifying Query Hints in a NamedQuery

Introduction to JPQL

Hibernate provides HSQL, while JDO-compliant providers such as BEA’s Kodo support JDO QL to query entities.

JPQL is an extension of EJB QL, the query language of EJB 2.

How is JPQL Different from SQL?

JPQL operates on classes and objects (entities) in the Java space. SQL operates on tables, columns, and rows in the database space.

While JPQL and SQL look similar to us, they operate in two very different worlds.

The JPQL Query Parser or Processor Engine of a persistence provider, as shown in below figure, translates the JPQL query into native SQL for the database being used by the persistence provider.

Defining and using SELECT

Defining UPDATE and DELETE

Identifying the query domain: naming an entity

What is a path expression?

Expressions such as c.categoryName and c.categoryId are known as path expressions

A path expression is an identifier variable followed by the navigation operator (.), and a persistence or association field.

An association field can contain either a single-value object or a collection. The association fields that represent one-to-many and many-to-many associations are collections of types, and such a path expression is a collection-value path expression.

Here c.items is collection-value path expression

If the association is either many-to-one or one-to-one, then the association fields are of a specific object type, and those types are known as single value path expressions

For example, c.items.user.firstName c.items.user.contactDetails.email

Conditional expressions and operators

A condition in the WHERE clause that filters results from a query is known as a conditional expression.

Operators supported by JPQL :

Using a range with BETWEEN

Using the IN operator

Using the LIKE operator

Dealing with null values and empty collections

You can use the IS NULL or IS NOT NULL operator to check whether a single-value path expression contains null or not null values.

For Example, WHERE c.parentCategory IS NOT NULL

You cannot use the IS NULL expression to compare a path expression that is of type collection. You have to use

IS [NOT] EMPTY

For Example, SELECT c FROM Category c WHERE c.items IS EMPTY

Checking for the existence of an entity in a collection

String functions

Arithmetic functions

JPQL temporal functions

Aggregate functions

Grouping with GROUP BY and HAVING

Ordering the query result

Using subqueries

Subqueries using ANY,ALL,SOME

Theta-joins

Inner Join

SELECT u FROM User u INNER JOIN u.Category c WHERE u.userId LIKE ?1

The INNER clause is optional.

Remember that when you use the JOIN operator by itself, an inner join is always performed

Outer joins

Fetch joins

Native SQL queries

Using dynamic queries with native SQL

Using a named native SQL query

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