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Oracle JPA Tips

Oracle Database Tips by Donald BurlesonJune 13, 2015

Question:  I understand that you can embed Oracle SQL inside the Java Persistence API (JPA), but I don't understand how JPA can interface with Oracle.  Can you explain how to execute a SQL statement inside JPA and show an example of executing an Oracle stored procedure with JPA?

Answer:  The Java Persistence API (JPA) allows you to execute Oracle SQL and gather the results in Java by binding the results to Java parameters.  The most complex aspect of using native SQL queries within JPA is binding results.  Binding a result set is simple when the query returns a single result of a simple type.  In the case where the result is a complex type you will have to create an entity to which you can map the native query, or define a complex result set mapped to multiple entities.

A simple example of using native SQL queries in JPA:

 List<Customer> customers = (List<Customer>)em.createNativeQuery
("SELECT * FROM customers", jpqlexample.entities.Customer.class)
.getResultList();
Iterator i = customers.iterator();
Customer cust;
out.println("Customers: " + "<br/>");
while (i.hasNext()) {
cust = (Customer) i.next();
out.println(cust.getCust_name() +"<br/>");
}

A big disadvantage of using native SQL queries is that your Java code becomes directly dependent on the database structure.  If you change the structure of the database, you will have to adjust your queries and recompile/redeploy your application.  You can, however, take advantage of Oracle stored procedures to move complex SQL statements to programs that are executed from within the database, and then calling those stored programs instead of making direct queries on the underlying tables.  With this structure, when you change the structure of the tables you must change the stored procedure instead of changing the actual Java code. 

An example of using a stored procedure called sum_total:

String sup_name ="Tortuga Trading";
BigDecimal sum =(BigDecimal)em.createNativeQuery ("SELECT sum_total(?1) FROM DUAL")
.setParameter(1, sup_name)
.getSingleResult();
out.println("The total cost of the ordered products
supplied by Tortuga Trading: " + sum +"");

sum_total is defined in the following example:

CREATE OR REPLACE FUNCTION sum_total(supplier VARCHAR2) RETURN NUMBER AS sup_sum NUMBER;
BEGIN
     SELECT
          SUM(p.price*l.quantity)
          INTO sup_sum
                FROM orders o
                JOIN orderlineitems l
                ON o.pono=l.pono
                JOIN products p
                ON l.prod_id=p.prod_id
                JOIN suppliers s
                ON p.sup_id=s.sup_id
                WHERE sup_name = supplier;
      RETURN sup_sum;
END;
/

For more information on creating stored procedures in Oracle, check out this helpful article.

 
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