Chapter 11. SQL Queries

11.1. Creating SQL Queries
11.2. Retrieving Persistent Objects with SQL

JPQL is a powerful query language, but there are times when it is not enough. Maybe you're migrating a JDBC application to JPA on a strict deadline, and you don't have time to translate your existing SQL selects to JPQL. Or maybe a certain query requires database-specific SQL your JPA implementation doesn't support. Or maybe your DBA has spent hours crafting the perfect select statement for a query in your application's critical path. Whatever the reason, SQL queries can remain an essential part of an application.

You are probably familiar with executing SQL queries by obtaining a java.sql.Connection, using the JDBC APIs to create a Statement, and executing that Statement to obtain a ResultSet. And of course, you are free to continue using this low-level approach to SQL execution in your JPA applications. However, JPA also supports executing SQL queries through the javax.persistence.Query interface introduced in Chapter 10, JPA Query. Using a JPA SQL query, you can retrieve either persistent objects or projections of column values. The following sections detail each use.

11.1. Creating SQL Queries

The EntityManager has two factory methods suitable for creating SQL queries:

public Query createNativeQuery (String sqlString, Class resultClass);
public Query createNativeQuery (String sqlString, String resultSetMapping);

The first method is used to create a new Query instance that will return instances of the specified class.

The second method uses a SqlResultSetMapping to determine the type of object or objects to return. The example below shows these methods in action.

Example 11.1. Creating a SQL Query

EntityManager em = ...;
Query query = em.createNativeQuery ("SELECT * FROM MAG", Magazine.class);
processMagazines (query.getResultList ());

In addition to SELECT statements, Kodo supports stored procedure invocations as SQL queries. Kodo will assume any SQL that does not begin with the SELECT keyword (ignoring case) is a stored procedure call, and invoke it as such at the JDBC level.


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