How to Resolve the 'Cannot Create TypedQuery for Query with More Than One Return' Error

What does the error 'Cannot create TypedQuery for query with more than one return' mean and how can I fix it?

// Example of a JPQL query that may cause this error
String jpql = "SELECT e FROM Employee e, Department d WHERE e.departmentId = d.id";

The error message 'Cannot create TypedQuery for query with more than one return' indicates that the Java Persistence API (JPA) cannot create a typed query because the result set contains multiple entity types or is not a single entity. This typically occurs when the query returns multiple entities or when you are attempting to retrieve multiple columns instead of a single entity instance.

// Correcting the query to return a single entity
String jpql = "SELECT e FROM Employee e WHERE e.departmentId = :departmentId";
TypedQuery<Employee> query = entityManager.createTypedQuery(jpql, Employee.class);

Causes

• The query returns multiple entities instead of a single entity instance.
• The result of the query does not match the entity class specified in the TypedQuery.
• Using a join or a complex JPQL that involves multiple tables without proper selection.

Solutions

• Ensure your query is selecting a single entity or a single scalar value.
• Use projections or specify a single return type if using multiple tables (e.g., DTOs).
• Double-check the JPQL syntax and ensure it returns a single entity type.

Mistake: Not specifying the proper entity type in TypedQuery after a join operation.

Solution: Always ensure that your query returns a single type matching the TypedQuery generic parameter.

Mistake: Returning multiple different entity classes in the same query.

Solution: Rework the query to either use separate queries for each entity or use DTOs.