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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Latest subquery specification
Date Fri, 07 Dec 2007 17:12:13 GMT
Subqueries
void addSubquery (Query subquery, String variableDeclaration,
	String candidateCollectionExpression);
This method adds a subquery to this query. A subquery is composed as  
a Query and subsequently attached to a different Query (the outer  
Query) by calling this method. The String parameters are trimmed of  
white space.
The Query parameter instance is unmodified as a result of the  
addSubquery or subsequent execution of the outer Query. Only some of  
the parameter query parts are copied for use as the subquery. The  
parts copied include the candidate class, filter, parameter  
declarations, variable declarations, imports, ordering specification,  
uniqueness, result specification, and grouping specification. The  
association with a PersistenceManager, the candidate collection or  
extent, result class, and range limits are not used.
The variableDeclaration parameter is the name of the variable  
containing the results of the subquery execution. If the same value  
of variableDeclaration is used to add multiple subqueries, the  
subquery replaces the previous subquery for the same named variable.  
If the subquery parameter is null, the variable is unset, effectively  
making the variable named in the variableDeclaration unbound. If the  
trimmed value is the empty String, or the parameter is null, then  
JDOUserException is thrown.
The candidateCollectionExpression is the expression from the outer  
query that represents the candidates over which the subquery is  
evaluated. If the trimmed value is the empty String, or the parameter  
is null, then the candidate collection is the extent of the candidate  
class.
Non-correlated subqueries
If the subquery has no references to expressions in the outer query  
the subquery is non-correlated.
For example, to find employees who work more than the average of all  
employees,
Query sub = pm.newQuery(Employee.class);
sub.setResult("avg(this.weeklyhours)");
Query q = pm.newQuery(Employee.class);
q.setFilter("this.weeklyHours > averageWeeklyhours");
q.addSubquery(sub, "double averageWeeklyhours", null);

Correlated subqueries
A correlated subquery is a subquery which contains references to  
expressions in the outer query. If the correlation can be expressed  
as a restriction of the candidate collection of the subquery, no  
parameters are needed.
For example, to find employees who work more than the average of  
their department employees:
Query sub = pm.newQuery(Employee.class);
sub.setResult("avg(this.weeklyhours)");
Query q = pm.newQuery(Employee.class);
q.setFilter("this.weeklyhours> averageWeeklyhours");
q.addSubquery(sub, "double averageWeeklyhours",  
"this.department.employees");

If the correlation cannot be expressed as a restriction of the  
candidate collection, the correlation is expressed as one or more  
parameters in the subquery which are bound to expressions of the  
outer query.
void addSubquery(String variableDeclaration,
	Query subquery, String candidateCollectionExpr, String parameter);
void addSubquery(String variableDeclaration,
	Query subquery, String candidateCollectionExpr, String[] parameters);
void addSubquery(String variableDeclaration,
	Query subquery, String candidateCollectionExpr, Map parameters);
The parameters in the above methods binds parameters in the subquery  
to expressions in the outer query. String parameters are trimmed of  
white space. The single String version of the method binds the named  
expression to the single parameter in the subquery. The String[]  
version of the method binds the named expressions in turn to  
parameters in the order in which they are declared in the subquery,  
or in the order they are found in the filter if not explicitly  
declared in the subquery. The Map version of the method treats the  
key of each map entry as the name of the parameter in the subquery,  
with or without the leading “:”, and the value as the name of the  
expression in the outer query. If the trimmed expression is the empty  
String for either the parameter or the value of the String[], or for  
any map key or value, that expression is ignored.
For example, to find employees who work more than the average of the  
employees in their department having the same manager:
Query sub = pm.newQuery(Employee.class);
sub.setResult("avg(this.weeklyhours)");
sub.setFilter("this.manager == :manager");
Query q = pm.newQuery(Employee.class);
q.setFilter("this.weeklyHours > averageWeeklyhours");
q.addSubquery(sub, "double averageWeeklyhours",  
"this.department.employees", "this.manager");
The parameter in the subquery “:manager” is bound to the expression  
“this.manager” in the context of the outer query.

Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
408 276-5638 mailto:Craig.Russell@sun.com
P.S. A good JDO? O, Gasp!


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