Enterprise JavaBeans

Enterprise JavaBeans

EJB Query Language

The Enterprise JavaBeans Query Language ("EJB QL") defines the queries for the finder and select methods of an entity bean with container-managed persistence.

Features:
1. EJB QL is a subset of SQL92
2. You define EJB QL queries in the deployment descriptor of the entity bean. Typically, a tool will translate these queries into the target language of the underlying data store. Because of this translation, entity beans with container-managed persistence are portable--their code is not tied to a specific type of data store.
　

Examples:

EJB QL statements are declared in the entity bean's deployment descriptor. The home interface of the Employee bean may be declared as follows:


public interface EmployeeHome extends 
javax.ejb.EJBHome {

      ...



      public Employee findByPrimaryKey(Integer id)

      throws RemoteException, CreateException;



      public Collection findByZipCode(String zipcode)

      throws RemoteException, CreateException;



}



The 


findByZipCode() 
method is used to obtain all the 




Employee 
beans with a certain zip code. That would be expressed by using the following 
EJB QL in the deployment descriptor. 









FROM contactInfo WHERE contactInfo.zip = ?1





This statement basically says "select all the 





Employee 
beans that have a zip code equal to the 




zipcode 
argument." The 




SELECT 
clause, which indicates what to select, is not needed in the EJB QL statements 
for find methods. That is because the find methods will always select the remote 
references of its own bean type. In that case, the select statement is supposed 
to return a collection of remote 




Employee 
bean references.