QueryDSL Window functions

Question

How can I write a query using window functions and selecting all fields in QueryDSL? In the docs there is an example like this:

query.from(employee)
.list(SQLExpressions.rowNumber()
    .over()
    .partitionBy(employee.name)
    .orderBy(employee.id));

but I need to generate a query like:

SELECT * FROM 
  (SELECT employee.name, employee.id, row_number() 
    over(partition BY employee.name
    ORDER BY employee.id)
  FROM employee) AS sub
WHERE row_number = 1

And is it possible to do it with JPAQuery?

Answer 1

JPAQuery supports only the expressivity of JPQL, so window functions are not supported, but paging should work using

query.from(employee).orderBy(employee.id).limit(1)

In case you need to use window functions and you need employee.name and employee.id out this should work

NumberExpression<Long> rowNumber = SQLExpressions.rowNumber()
    .over()
    .partitionBy(employee.name)
    .orderBy(employee.id).as("rowNumber");

query.select(employee.name, employee.id)
    .from(SQLExpressions.select(employee.name, employee.id, rowNumber)
                        .from(employee).as(employee))
    .where(Expressions.numberPath(Long.class, "rowNumber").eq(1L))
    .fetch();

Answer 2

As written by @timo Window functions (rank, row_number) are not supported by JPQL (JPA 2.1 version) and hence by JPAQuery (QueryDsl Jpa 4.1.4).

You can however rewrite your query so that is does not use rank over():

select a.* from employees a
where
(
    select count(*) from employees b
    where 
       a.department = b.department and
       a.salary <= b.salary
) <= 10
order by salary DESC

This is supported by JPAQuery, it probably goes like this.

final BooleanBuilder rankFilterBuilder = 
    new BooleanBuilder(employee.department.eq(employee2.department));
rankFilterBuilder.and(employee.salary.loe(employee2.salary));

query.from(employee)
.where(JPAExpressions.selectFrom(employee2)
            .where(rankFilterBuilder)
            .select(employee2.count())
            .loe(10))
.orderBy(employee.salary);