Similar to using tables, you can give different correlation names to multiple instances of a common table expression. This permits you to join a common table expression to itself. For example, the query below produces pairs of departments that have the same number of employees, although there are only two departments with the same number of employees in the SQL Anywhere sample database.

WITH CountEmployees( DepartmentID, n ) AS
    ( SELECT DepartmentID, COUNT( * ) AS n
      FROM Employees GROUP BY DepartmentID )
SELECT a.DepartmentID, a.n, b.DepartmentID, b.n
FROM CountEmployees AS a JOIN CountEmployees AS b
ON a.n = b.n AND a.DepartmentID < b.DepartmentID;

 See also