When you specify a NATURAL JOIN and put a join condition in an ON clause, the result is the conjunction of the two join conditions.

For example, the following two queries are equivalent. In the first query, SQL Anywhere generates the join condition Employees.DepartmentID = Departments.DepartmentID. The query also contains an explicit join condition.

SELECT GivenName, Surname, DepartmentName
FROM Employees NATURAL JOIN Departments
  ON Employees.ManagerID = Departments.DepartmentHeadID;

The next query is equivalent. In it, the natural join condition that was generated in the previous query is specified in the ON clause.

SELECT GivenName, Surname, DepartmentName
FROM Employees JOIN Departments
  ON Employees.ManagerID = Departments.DepartmentHeadID
   AND Employees.DepartmentID = Departments.DepartmentID;