'WITH' clause does not work with in operator

Question

I have a simple query:

with MaxSal AS (SELECT MAX(Salary), DepartmentId FROM Employee GROUP BY DepartmentId)
SELECT Department.Name AS "Department", Employee.Name AS "Employee", Employee.Salary AS "Salary" FROM Employee
JOIN Department ON Department.Id = Employee.DepartmentId
WHERE (Employee.Salary, Employee.DepartmentId) in MaxSal;

It should create a MaxSal temporary table and then in the WHERE section it should check whether the Salary, DepId are inside said table. Unfortunately, this query gives me ORA-00920: invalid relational operator which I guess is referring to the in operator. If I put the Sub-query directly instead of first putting it in a temp table, everything works as expected.
Can somebody tell me why it isn't working with the WITH clause?

Answer 1

You need a table reference to refer to the CTE and that, in turn, requires a FROM clause. You can do what you want using a subquery

WITH MaxSal AS (
      SELECT MAX(Salary) as max_salary, DepartmentId
      FROM Employee
      GROUP BY DepartmentId
     )
SELECT d.Name AS Department, e.Name AS Employee, 
       e.Salary AS Salary
FROM Employee e JOIN
     Department d
     ON d.Id = e.DepartmentId
WHERE (e.Salary, e.DepartmentId) in (SELECT max_salary, DepartmentId) FROM MaxSal);

That said, RANK() is much simpler:

SELECT Department, Name AS Employee, Salary
FROM (SELECT e.*, d.Name as Department,
             RANK() OVER (PARTITION BY d.id ORDER BY e.Salary DESC) as seqnum
      FROM Employee e JOIN
           Department d
           ON d.Id = e.DepartmentId
     ) ed
WHERE seqnum = 1;