Is Select EXISTS() possible in JPQL?

Question

I'm trying to run a query that checks if some conditions are true and returns a simple boolean result as output. What makes it slightly tricky is that one of the conditions is to test for whether no results are returned for a set of criteria.

I'm currently using JPA-2.0 with hibernate as my provider, backed by MySQL. I have gotten an example query working fine in MySQL, but when trying to get it running in JPQL it flops. The MySQL query looks a bit like this:

Select exists(Select statement with criteria) 
  or not exists(Select statement with criteria);

I also got the same output using CASE, but as JPQL doesn't support that statement.

Anyways, when I try to use a similar query in JPQL I get the error:

"unexpected end of subtree"

which from my understanding means that something is missing in the query. Does anyone have any idea how to fix it?

Answer 1

You can do a boolean query using a case expression.

As of JPA 2.0 (Java EE 6) you can create a TypedQuery .

String query = "select case when (count(*) > 0)  then true else false end from ......"
TypedQuery<Boolean> booleanQuery = entityManager.createQuery(query, Boolean.class);
boolean exists = booleanQuery.getSingleResult();

In JPA 1.0 (Java EE 5) you must use an untyped query.

Query booleanQuery = entityManager.createQuery(query);
boolean exists = (Boolean) booleanQuery.getSingleResult();