What are the Alternatives to UNION Queries in Hibernate?

Hibernate Union Alternatives

While Hibernate does not natively support union queries, there are alternative approaches to achieve similar functionality:

• Subqueries using in: Form an expression using id in (subquery1) or id in (subquery2) to retrieve results from multiple subqueries.

Example:

from Person p
  where p.id in (select p1.id from Person p1 where p1.name="Joe")
    or p.id in (select p2.id from Person p2 join p2.children c where c.name="Joe");

Drawback: May result in performance issues in MySQL due to lack of index utilization.

• Manual Join and Deduplication: Execute two separate queries and manually combine the results into a unique set or list to remove duplicates.

Example:

// use set for uniqueness
Set<Person> people = new HashSet<Person>((List<Person>) query1.list());
people.addAll((List<Person>) query2.list());
return new ArrayList<Person>(people);

Considerations:

• Using multiple subqueries can be more efficient than one complex query.
• Manual joins and deduplication may introduce more overhead compared to native UNION.
• Selecting a suitable alternative depends on factors such as performance requirements, database type, and data distribution.

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