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 subqueriesIf 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 subqueriesA 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:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!
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