using subqueries in jpa criteria api

Question

I\'m studying JPA criteria api and my database contains Employee table. I am trying to find all the employees who are paid second highest salary. I was able to write JPQL su

Answer 1

After some more trial and error, I could write the query to select employees with second maximum salary. I would like to suggest that you should write a JPQL query first and write the criteria api accordingly. This is what I analyzed from JPQL.

SELECT e FROM Employee e 
WHERE e.salary = (SELECT MAX(emp.salary) FROM Employee emp 
WHERE emp.salary < (SELECT MAX(employee.salary) FROM Employee employee) )

Now we can see that

• There are 2 subqueries, i.e. subquery of main query contains another subquery
• The identification variables e, emp and employee correspond to the main query, subquery of main query and subquery of subquery.
• Now while comparing the result of subqueries i.e. maximum salary compared with the employee salary of outer query, the identification variable from outer query is used. for e.g. WHERE emp.salary = (SELECT MAX(emp.salary) FROM Employee emp)

Now let us convert this query in criteria api.

First write CriteriaQuery that corresponds to outermost query i.e. SELECT e FROM Employee e WHERE e.salary =

CriteriaQuery<Employee> c1 = cb.createQuery(Employee.class);
Root<Employee> e3 = c1.from(Employee.class);
c1.select(e3);

Let us leave the WHERE e.salary = for now and go for the subquery

Now this should have a subquery that selects the maximum salary of employees i.e. SELECT MAX(emp.salary) FROM Employee emp WHERE emp.salary < again let us leave the WHERE emp.salary < for now.

Subquery<Long> sq1 = c1.subquery(Long.class);
Root<Employee> e4 = sq1.from(Employee.class);
sq1.select(cb.max(e4.<Long> get("salary")));

repeating this for subquery of above subquery,

Subquery<Long> sq2 = sq1.subquery(Long.class);
Root<Employee> e5 = sq2.from(Employee.class);
sq2.select(cb.max(e5.<Long> get("salary")));

Now we have written subqueries but WHERE conditions need to be applied yet. So now the where condition in criteria api corresponding to WHERE emp.salary < (SELECT MAX(employee.salary) FROM Employee employee) will be as below.

sq1.where(cb.lessThan(e4.<Long> get("salary"), sq2));

Similarly, WHERE condition corresponding to WHERE e.salary = (SELECT MAX(emp.salary) FROM Employee emp will be as below.

c1.where(cb.equal(e3.<Long> get("salary"), sq1));

So the complete query which gives the employees with second highest salary can be written in criteria api as below.

        CriteriaQuery<Employee> c1 = cb.createQuery(Employee.class);
    Root<Employee> e3 = c1.from(Employee.class);
    c1.select(e3);

    Subquery<Long> sq1 = c1.subquery(Long.class);
    Root<Employee> e4 = sq1.from(Employee.class);
    sq1.select(cb.max(e4.<Long> get("salary")));

    Subquery<Long> sq2 = sq1.subquery(Long.class);
    Root<Employee> e5 = sq2.from(Employee.class);
    sq2.select(cb.max(e5.<Long> get("salary")));

    sq1.where(cb.lessThan(e4.<Long> get("salary"), sq2));
    c1.where(cb.equal(e3.<Long> get("salary"), sq1));

    employees = em.createQuery(c1).getResultList();

    for (Employee employee : employees) {
        System.out.println(employee.getName() + " " + employee.getSalary());
    }

Answer 2

As documented, it cannot work because Number is not Comparable:

<Y extends java.lang.Comparable<? super Y>> Predicate lessThan(Expression<? extends Y> x,
                                                               Expression<? extends Y> y)

For expression with Number there is method Criteriabuilder.lt that takes such arguments:

c.where(cb.lt(e2.<Number>get("salary"), e3.<Number>get("salary")));

Other option is to change type argument from Number to something more specific. If salary is Long, following should work:

Subquery<Long> sq = c.subquery(Long.class);
Root<Employee> e2 = sq.from(Employee.class);
sq.select(cb.max(e2.<Long> get("salary")));

Subquery<Long> sq1 = sq.subquery(Long.class);
Root<Employee> e3 = sq1.from(Employee.class);
sq1.select(cb.max(e3.<Long> get("salary")));

c.where(cb.lessThan(e2.<Long>get("salary"), e3.<Long>get("salary")));
c.where(cb.equal(e1.get("salary"), sq));