The WINDOW clause allows you to refer to a window by name when you use a ROW NUMBER function in a selectExpression.
See ROW_NUMBER function and selectExpression for more information.
WINDOW windowName AS windowSpecification
In a WINDOW clause, windowName is a SQLIdentifier.
Currently, the only valid windowSpecification is a set of empty parentheses (()), which indicates that the function is evaluated over the entire result set.
SELECT ROW_NUMBER() OVER R, B, SUM(A) FROM T5 GROUP BY B WINDOW R AS ()