web server - Borland Technical Publications

Sub-Queries
We can get the average salary of each department using a single query method:
SELECT e.DEPARTMENT, AVG(e.SALARY) FROM EMPLOYEE e GROUP BY e.DEPARTMENT
The results are:
DEPARTMENT
AVG(SALARY)
Coffee Room 950
Mail Room 1100
The Closet with the Light Out 850
Sub-Queries
Dynamic Queries
Sub-queries are permitted as deep as the database implementation being queried
allows. For example, you could use the following sub-query (in bold) specified in ejbjar.xml.
Note that the sub-query includes ORDER BY as well, and the results are to be
returned in descending (DESC) order.
findApStatisticsWithGreaterThanAverageValue
SELECT Object(s1) FROM ApStatistics s1 WHERE s1.averageValue >
SELECT AVG(s2.averageValue) FROM ApStatistics s2 ORDER BY s1.averageValue
DESC
See your database implementation documentation for details on the appropriate use of
sub-queries.
There are situations where you may need to search dynamically for data, based on
variable criteria. Unfortunately EJB-QL queries do not support this scenario. Since
EJB-QL queries are specified in the deployment descriptor, any changes to the queries
require re-deployment of the bean. The Borland Enterprise Server offers a Dynamic
Query feature which allows you to construct and execute EJB-QL queries dynamically
and programmatically in the bean code.
Dynamic queries offer these benefits:
■
allow you to create and execute new queries without having to update and deploy
an EJB.
■
reduce the size of the EJB's deployment descriptor file because finder queries can
be dynamically created instead of statically defined in the deployment descriptors.
Dynamic queries don't need to be added to the deployment descriptor. They are
declared in the bean class for dynamic ejbSelects, or in the local or remote home
interfaces for dynamic finders.
A finder method for a dynamic query is:
public java.util.Collection findDynamic(java.lang.String ejbql, Class[]
types, Object[] args)
throws javax.ejb.FinderException
public java.util.Collection findDynamic(java.lang.String ejbql, Class[]
types, Object[] args, java.lang.String sql)
throws javax.ejb.FinderException
Chapter 17: EJB-QL and Data Access Support 157