When a join statement selects from three or more tables, Derby analyzes any equijoin predicates between simple column references within each query block and adds additional equijoin predicates where possible if they do not currently exist. For example, Derby transforms the following query:

SELECT * FROM samp.employee e, samp.emp_act a, samp.emp_resume r
WHERE e.empno = a.empno
and a.empno = r.empno

into the following:

SELECT * FROM samp.employee e, samp.emp_act a, samp.emp_resume r
WHERE e.empno = a.empno
and a.empno = r.empno
and e.empno = r.empno