PERCENT_RANK( ) OVER ( window-spec )
计算百分比，会进行迭代，计算方法为 (RANK - 1)/(n-1)。
例：
(1) SELECT DepartmentID, Surname, Salary, Sex,
PERCENT_RANK() OVER ( PARTITION BY Sex
ORDER BY Salary DESC ) AS PctRank
FROM Employees
WHERE State IN ( 'NY' );
(2)SELECT Surname, Salary,
PERCENT_RANK () OVER ( ORDER BY Salary DESC ) "Rank"
FROM Employees
WHERE State IN ( 'UT', 'AZ' ) AND Sex IN ( 'F' );
(3)SELECT *
FROM ( SELECT Surname, Salary,
PERCENT_RANK () OVER ( ORDER BY Salary DESC ) "Rank"
FROM Employees
WHERE Sex IN ( 'M' ) )
AS DerivedTable ( Surname, Salary, Percent )
WHERE Percent < 0.05;
而ratio_to_report(ziduan) OVER ()查询的是占总量的百分比，不会进行迭代计算。
