UNION clause operation
The UNION clause specifies a combination of the output of two subqueries. The two queries must return the same number of elements and compatible types.
For the purposes of UNION, all Enterprise JavaBeans (EJB) types in the same inheritance hierarchy
are considered compatible. UNION requires that equality be defined for the element
types.
query_expression := query_term [UNION [ALL] query_term]*
query_term := {select_clause_dynamic from_clause [where_clause]
[group_by_clause] [having_clause] } | (query_expression) }
You cannot use dependent value objects with UNION.
UNION ALL combines all results together in a single collection.
UNION combines results but eliminates duplicates.
If ORDER BY is used together with UNION, the ORDER BY must refer to selection expression using integer numbers.
Examples: UNION operation
This example returns a collection of all employee objects of type EmpBean and all manager objects
of type ManagerBean where ManagerBean is a subtype of
EmpBean.
select e from EmpBean e union all select m from DeptBean d, in(d.mgr) m
This example shows a query that is not valid, because EmpBean and DeptBean are not
compatible.
select e from EmpBean e union all select d from DeptBean d