HQL Unions

Support for set operations (UNION, INTERSECT, EXCEPT) was added in Hibernate 6.0.

The #unionQuery is interesting idea.

I do think it will be possible to implement UNION as a HQL language feature though given the design of SQM and the SQL AST (internal “query syntax trees”). Possibly even using the support for CTE statements - see org.hibernate.query.sqm.tree.cte.SqmCteStatementand org.hibernate.sql.ast.tree.cte.CteStatement

As a suggestion... you can skip HQL validation by adding an intermediate function called "unionQuery", the suggested sintax would be like this:

That way you can reuse the same HQL validation and the only left thing to do is to validate each query returned values/types;

I fixed this for my requirement by now doing the following (i'm simplifying a lot this example, is only to give you the idea):

In the meantime you could also try out Blaze-Persistence if you really need support for SET functions but don't want to leave your domain model thinking. It works on top of JPA but the integration is currently for Hibernate only.

If UNION support makes it into HQL grammars, it will happen as part of the work being done under https://github.com/hibernate/hibernate-semantic-query which has been discussed alot on the hibernate-dev mailing list. If anyone is interested in helping with that effort it will help get things like this implemented sooner. If not, it gets done as I have time between fixing bugs and other work.

In the meantime, per the incomplete support for UNION in the current grammars will be removed in 5.0.4. Its incomplete and causes problems in recognizing certain valid queries.