<?xml version="1.0" encoding="ISO-8859-1"?>
<document url="http://castor.exolab.org/howto/how-to-use-limit-clause.xml">
  <properties>
    <title>How to use a LIMIT clause with OQL</title>
    <abstract>
       General information about using a LIMIT clause with OQL
    </abstract>
    <status>Final</status>
  </properties>
  <body>
    <title>How to use a LIMIT claue with OQL</title>
    <header>
    </header>

	<section title="Overview">

        <p></p>
        
    </section>
    
    <section title="Intended Audience">

        <p>Anyone who wants to execute an OQL statement and limit the result size.</p>
        
        <p>The example given describes the addition of LIMIT/OFFEST clauses to an 
           existing OQL statement.</p>
        
    </section>
    
    <section title="Prerequisites">

        <p>You should have a valid class mapping for two Java classes <tt>Product</tt> and 
        <tt>ProductGroup</tt>, similar to the following 
        one:</p>

                <code-panel>
package myapp;

public class Product 
{
    private int       _id;

    private String    _name; 

    private float     _price; 

    private ProductGroup _group;


    public int getId() { ... }

    public void setId( int anId ) { ... }

    public String getName() { ... }

    public void setName( String aName ) { ... }

    public float getPrice() { ... }

    public void setPrice( float aPrice ) { ... }

    public ProductGroup getProductGroup() { ... }

    public void setProductGroup( ProductGroup aProductGroup ) { ... }
}
               </code-panel>

      <p>The following fragment shows the Java class declaration for the ProductGroup class:</p>
      
                    <code-panel>


public class ProductGroup
{

    private int       _id;

    private String    _name;

    public int getId() { ... }

    public void setId( int id ) { ... }

    public String getName() { ... }

    public void setName( String name ) { ... }

}
                </code-panel>
            
    </section>

    <section title="Steps">

        <p>Here is how to proceed.</p>

        <section title="Compose an OQL statement to obtain all ProductGroup instances">

            <p>The following code fragment shows an OQL query to select the all 
            ProductGroup instances.</p>
                
            <code-panel>
OQLQuery query = db.getOQLQuery("select product from ProductGroup product");
query.bind(10);
OQLResults results = query.execute();
            </code-panel>
            
        </section>
        
        <section title="Add LIMIT clause to OQL statement">

            <p>The following code fragment shows the same OQL query as above, to this time 
            the LIMIT keyword is added to select the first 10 ProductGroup instances only.</p>

            <code-panel>
OQLQuery query = db.getOQLQuery("select product from ProductGroup product <b><font color="red">LIMIT $1</font></b>");
query.bind(10);
OQLResults results = query.execute();
            </code-panel>

        </section>

        <section title="Add OFFSET clause to OQL statement">

            <p>Below is the same OQL query again, restricting the number of ProductGroup instances
            returned to 10, though this time it is specified that the ProductGroup 
            instances 11 to 20 should be returned.</p>

            <code-panel>
OQLQuery query = db.getOQLQuery ("select product from ProductGroup as product <b>LIMIT $1 <font color="red">OFFSET $2</font></b>");
query.bind(10);
OQLResults results = query.execute();
            </code-panel>


        </section>

    </section>
    
    <section title="Limitations">
        
         <p>The following RDBMS fully/partially support LIMIT/OFFSET clauses.</p>

         <table border="0" cellspacing="1" cellpadding="2" bgcolor="#7270c2">
            <tr>
                <td>
                    <table border="0" cellspacing="1" cellpadding="8" bgcolor="#ededed">
                        <tr bgcolor="#7270c2" style="color:#FFFFFF">
                            <th align="left">RDBMS</th><th>LIMIT</th><th>OFFSET</th>
                        </tr>
                        <tr>
                            <td>postgreSQL</td><td>Yes</td><td>Yes</td>
                        </tr>
                        <tr>
                            <td>mySQL</td><td>Yes</td><td>Yes</td>
                        </tr>
                        <tr>
                            <td>Oracle - 1)2)</td><td>Yes</td><td>Yes</td>
                        </tr>
                        <tr>
                            <td>HSQL</td><td>Yes</td><td>Yes</td>
                        </tr>
                        <tr>
                            <td>MS SQL</td><td>Yes</td><td>-</td>
                        </tr>
                        <tr>
                            <td>DB2</td><td>Yes</td><td>-</td>
                        </tr>
                   </table>
               </td>
            </tr>
         </table>

         <p>1) Oracle has full support for LIMIT/OFFSET clauses for release 8.1.6 and
            later.</p>
         <p>2) For the LIMIT/OFFSET clauses to work properly the OQL query is required
			to include a ORDER BY clause.</p>

        
    </section>
    
    <section title="Tips">

        <ul>
            <li>In the case a RDBMS does not support LIMIT/OFFSET clauses, a 
                <tt>SyntaxNotSupportedException</tt> will be thrown.</li>
                
        </ul>    
        
    </section>


    <section title="References">

        <ul>
            <li><a href="oql.html">Castor JDO's OQL</a></li>
        </ul>
        
    </section>


    </body>
</document>