Revision as of 14:01, 4 October 2011

Design Specification: Enhanced JPQL

ER 312146

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Project overview

JPQL currently offers a sub-set of SQL functionality. Some queries that are possible using SQL cannot be defined using JPQL.

It is desired to make EclipseLink's JPQL support be more complete of what is possible in SQL.

A key requirement from users (#1 voted enhancement) is to have ON clause support in JPQL.

Other missing features of SQL could also be added to JPQL, such as sub selects in the SELECT from FROM clauses, and enhanced function support.

Other JPQL enhancements:

• allow qualified class names in from clause
• allow alias on join fetch
• support for cast function
• group by fails in in
• support union and intersect
• support platform independent functions and custom operators
• support nested join fetches
• support connect by
• support common functions
• JPA 2.1 sub select in functions, having, select, from

Other JPQL bugs:

• incorrect SQL for distinct
• join to element collection does not work
• join ignored
• group by ignored [1]
• duplicate join in sub select
• package with order fails to parse
• stack overflow parsing query
• join of map fails
• plus not aliasable
• sub-string fails
• element collection fails with in
• count distinct fails
• bad sql for in [2] [3]
• jpql sub selects incorrect
• member of fails
• concat fails in subquery
• support inner joins in from clause
• expression order in jpql causes error
• join fetch ignored
• dot removes outer join
• is not null should be used
• [ from clause not used in select or where is omitted]

Concepts

ON clause : the SQL clause part of the FROM clause that defines how to tables are joined, this can be used for both joins and outer joins, but is required for outer joins.

outer join : A join where if a row in the source table has no joined rows in the target table it is still included in the join result with a null row for the target table.

ANTRL : Third party library currently used in EclipseLink for parsing JPQL.

Hermes : New JPQL parser developed by Dali project for parsing JPQL at design time. Currently included in EclipseLink SVN, but not currently used.

Requirements

• Support additional ON clause for a relationship join.
• Support an ON clause on the join for two independent objects.

Design Constraints

• Outer join capabilities differ in different databases.
• SQL capabilities differ in different databases.

Functionality

• Extend ObjectExpression to support an onClause, must normalize and copy expression accordingly.
• Allow ExpressionBuilders to be added to nonFetchJoinExpressions in ObjectLevelReadQuery.

Testing

ON clause needs Expression tests in core testing and JPQL tests in JPA testing. Testing both 1-1, 1-m, relationship and parallel.

API

JPQL

• Select e from Employee e left join e.address a on (a.city = 'Ottawa')
• Select e from Employee e left join Project p on (p.teamLeader = e)

Native API

• Expression
  • on(Expression onClause)
• ObjectLevelReadQuery
  • addNonFetchJoin(Expression)
  • addLeftNonFetchJoin(Expression)

Examples:

ExpressionBuilder employee = new ExpressionBuilder();
Expression address = employee.getAllowingNull("address");
address.on(address.get("city").equal("Ottawa"));
ReadAllQuery query = new ReadAllQuery(Employee.class, employee);
query.addNonFetchJoin(address);
 
ExpressionBuilder employee = new ExpressionBuilder(Employee.class);
ExpressionBuilder project = new ExpressionBuilder(Project.class);
project.on(project.get("teamLeader").equal(employee));
ReadAllQuery query = new ReadAllQuery(Employee.class, employee);
query.addNonFetchJoin(project);

Config files

Documentation

Need to document our BNF, example queries and support beyond JPA 2.0/2.1 in query section.

Open Issues

Decisions

Future Considerations

• Other JPQL enhancements.
• Criteria support.