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| <title>2. Retrieving Persistent Objects with SQL</title><base href="display"><link rel="stylesheet" type="text/css" href="css/docbook.css"><meta name="generator" content="DocBook XSL Stylesheets V1.79.1"><link rel="home" href="manual.html" title="Apache OpenJPA 2.4 User's Guide"><link rel="up" href="jpa_overview_sqlquery.html" title="Chapter 12. SQL Queries"><link rel="prev" href="jpa_overview_sqlquery.html" title="Chapter 12. SQL Queries"><link rel="next" href="jpa_overview_mapping.html" title="Chapter 13. Mapping Metadata"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">2. |
| Retrieving Persistent Objects with SQL |
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| SQL Queries |
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| Retrieving Persistent Objects with SQL |
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| <a class="indexterm" name="d5e5354"></a> |
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| <p> |
| When you give a SQL <code class="classname">Query</code> a candidate class, it will |
| return persistent instances of that class. At a minimum, your SQL must select |
| the class' primary key columns, discriminator column (if mapped), and version |
| column (also if mapped). The JPA runtime uses the values of the primary key |
| columns to construct each result object's identity, and possibly to match it |
| with a persistent object already in the <code class="classname">EntityManager</code>'s |
| cache. When an object is not already cached, the implementation creates a new |
| object to represent the current result row. It might use the discriminator |
| column value to make sure it constructs an object of the correct subclass. |
| Finally, the query records available version column data for use in optimistic |
| concurrency checking, should you later change the result object and flush it |
| back to the database. |
| </p> |
| <p> |
| Aside from the primary key, discriminator, and version columns, any columns you |
| select are used to populate the persistent fields of each result object. JPA |
| implementations will compete on how effectively they map your selected data to |
| your persistent instance fields. |
| </p> |
| <p> |
| Let's make the discussion above concrete with an example. It uses the following |
| simple mapping between a class and the database: |
| </p> |
| <div class="mediaobject"><table border="0" summary="manufactured viewport for HTML img" style="cellpadding: 0; cellspacing: 0;" width="213"><tr><td><img src="img/sqlquery-model.png"></td></tr></table></div> |
| <div class="example" id="jpa_overview_sqlquery_objex"><p class="title"><b>Example 12.2. |
| Retrieving Persistent Objects |
| </b></p><div class="example-contents"> |
| |
| <pre class="programlisting"> |
| Query query = em.createNativeQuery("SELECT ISBN, TITLE, PRICE, " |
| + "VERS FROM MAG WHERE PRICE > 5 AND PRICE < 10", Magazine.class); |
| List<Magazine> results = (List<Magazine>) query.getResultList(); |
| for (Magazine mag : results) |
| processMagazine(mag); |
| </pre> |
| </div></div><br class="example-break"> |
| <p> |
| The query above works as advertised, but isn't very flexible. Let's update it to |
| take in parameters for the minimum and maximum price, so we can reuse it to find |
| magazines in any price range: |
| </p> |
| <div class="example" id="jpa_overview_sqlquery_obj_paramex"><p class="title"><b>Example 12.3. |
| SQL Query Parameters |
| </b></p><div class="example-contents"> |
| |
| <pre class="programlisting"> |
| Query query = em.createNativeQuery("SELECT ISBN, TITLE, PRICE, " |
| + "VERS FROM MAG WHERE PRICE > ?1 AND PRICE < ?2", Magazine.class); |
| |
| query.setParameter(1, 5d); |
| query.setParameter(2, 10d); |
| |
| List<Magazine> results = (List<Magazine>) query.getResultList(); |
| for (Magazine mag : results) |
| processMagazine(mag); |
| </pre> |
| </div></div><br class="example-break"> |
| <p> |
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| Like JDBC prepared statements, SQL queries represent parameters with question |
| marks, but are followed by an integer to represent its index. |
| </p> |
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| SQL Queries |
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| Mapping Metadata |
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