JPQL equivalent of SQL query using unions and selecting constants

Question

I\'ve written a SQL query that basically selects from a number of tables to determine which ones have rows that were created since a particular date. My SQL looks something

Answer 1

I did a little more searching and found a (seemingly obscure) feature of JPA that serves my purpose perfectly. What I found is that JPA 2 has a type keyword that allows you to limit polymorphic queries to a particular subclass, like so:

SELECT widget
FROM BaseWidget widget
WHERE TYPE(widget) in (WidgetB, WidgetC)

I've found that JPA (or at least Hibernate as a JPA implementation) allows you to use type not only in constraints but also in select lists. This is approximately what my query ended up looking like:

SELECT DISTINCT TYPE(widget)
FROM BaseWidget widget
WHERE widget.creationTimestamp > :cutoff

That query returns a list of Class objects. My original query was selecting string literals because that's closest to what I might have done in SQL. Selecting Class is actually preferable in my case. But if I did prefer to select a constant based on an entity's type, that is the exact scenario that Oracle's documentation uses to illustrate case statements:

SELECT p.name
CASE TYPE(p)
  WHEN Student THEN 'kid'
  WHEN Guardian THEN 'adult'
  WHEN Staff THEN 'adult'
  ELSE 'unknown'
END
FROM Person p

Answer 2

Some JPA providers do support UNION,

http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Querying/JPQL#UNION

but your query seems very complex, and non object-oriented, so using a native SQL query would probably be best.