How Can I Achieve the Equivalent of UNION Queries in Hibernate?

Alternatives to Hibernate Union Queries

Despite the absence of native union query support in Hibernate, there are several methods to achieve similar results.

Subselects:

One approach is to utilize subselects within a single query. This involves specifying multiple id in (subquery) conditions connected by or operators. However, it's worth noting that this method can lead to performance limitations in MySQL.

Example:

Query query1 = entityManager.createQuery(
    "from Person p where p.name = 'Joe'", Person.class);

Query query2 = entityManager.createQuery(
    "from Person p join p.children c where c.name = 'Joe'", Person.class);

Set<Person> people = new HashSet<>();

people.addAll((List<Person>) query1.list());
people.addAll((List<Person>) query2.list());

List<Person> result = new ArrayList<>(people);

View Tables:

Another option is to create a view table containing the desired result set. This approach requires some database administration work, but can provide performance benefits.

Plain JDBC:

While plain JDBC allows for direct union queries, it sacrifices the convenience of Hibernate query annotations and validation.

Other Suggestions:

• Using a poor man's join, which involves executing two separate queries and combining the results in your code.
• Considering the option of a more complex, single query instead of multiple, simpler ones.

Performance Considerations:

When using subselects, the performance impact of the resulting MySQL query should be taken into account. Without appropriate indexing, the query may struggle with large datasets.

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