JDOQL

JDOQL, the Java Data Objects Query Language, is the standard query language for Java Data Objects (JDO). It enables applications to query persistence-capable objects stored in a datastore. JDOQL operates on the persistent fields and relationships of registered classes rather than on database tables and columns. The syntax resembles SQL but uses Java class names and property names. A typical query begins with SELECT FROM <ClassName> and may include a WHERE clause and an ORDER BY clause, with optional use of parameters.

Queries are created and executed through the JDO API via a PersistenceManager. A query targets a candidate class, and conditions in the WHERE clause reference field values, object relationships, and collection contents. Parameter binding is supported, usually by declaring parameter types and supplying values at execution time.

Example: SELECT FROM com.example.Person WHERE age > 21

This returns instances of com.example.Person satisfying the predicate. Another form uses parameters: declareParameters("int minAge"); SELECT FROM com.example.Person WHERE age > minAge; Collection<Person> results = (Collection<Person>) q.execute(18);

JDOQL is designed to be portable across JDO implementations and datastores that support JDO. It emphasizes object navigation over SQL-style joins, with joins expressed by traversing object relationships in path expressions. It also supports querying on fields within related objects and on collection-valued fields.

Limitations and extensions vary by implementation; some support aggregates, grouping, and distinct results, while others focus on basic filtering and ordering. For specifics, consult the JDO specification and the documentation of the particular JDO implementation in use.