Java Persistent Objects - JPA Queries

The JPA specification defines its interpretation of SQL, for selecting objects from the datastore. To provide a simple example, this is what you would do

Query q = em.createNativeQuery("SELECT p.id, o.firstName, o.lastName FROM Person p, Job j " +
                "WHERE (p.job = j.id) AND j.name = 'Cleaner'");
List results = (List)q.getResultsList();

This finds all "Person" objects that do the job of "Cleaner". The syntax chosen has to be runnable on the RDBMS that you are using (and since SQL is anything but "standard" you will likely have to change your query when moving to another datastore).

Input Parameters

In JPQL queries it is convenient to pass in parameters so we dont have to define the same query for different values. Here's an example

Numbered Parameters :
Query q = em.createQuery("SELECT p FROM Person p WHERE p.lastName = ?1 AND p.firstName = ?2");
q.setParameter(1, theSurname);
q.setParameter(2, theForename);

So we have parameters that are prefixed by ? (question mark) and are numbered starting at 1. We then use the numbered position when calling Query.setParameter(). With SQL queries we can't use named parameters. This is known as numbered parameters.

JPOX also supports use of named parameters where you assign names just like in JPQL. This is not defined by the JPA specification so dont expect other JPA implementations to support it. Let's take the previous example and rewrite it using named parameters, like this

Named Parameters :
Query q = em.createQuery("SELECT p FROM Person p WHERE p.lastName = :firstParam AND p.firstName = :otherParam");
q.setParameter("firstParam", theSurname);
q.setParameter("otherParam", theForename);

Range of Results

With SQL you can select the range of results to be returned. For example if you have a web page and you are paginating the results of some search, you may want to get the results from a query in blocks of 20 say, with results 0 to 19 on the first page, then 20 to 39, etc. You can facilitate this as follows

Query q = em.createNativeQuery("SELECT p FROM Person p WHERE p.age &gt; 20");
q.setFirstResult(0);
q.setMaxResults(20);

So with this query we get results 0 to 19 inclusive.

Query Execution

There are two ways to execute a SQL query. When you know it will return 0 or 1 results you call

Object result = query.getSingleResult();

If however you know that the query will return multiple results, or you just don't know then you would call

List results = query.getResultList();