PERCENT RANK() در sql server چیست و چه کاری انجام می دهد

USE AdventureWorks2012;  
GO  
SELECT Department, LastName, Rate,   
       CUME_DIST () OVER (PARTITION BY Department ORDER BY Rate) AS CumeDist,   
       PERCENT_RANK() OVER (PARTITION BY Department ORDER BY Rate ) AS PctRank  
FROM HumanResources.vEmployeeDepartmentHistory AS edh  
    INNER JOIN HumanResources.EmployeePayHistory AS e    
    ON e.BusinessEntityID = edh.BusinessEntityID  
WHERE Department IN (N'Information Services',N'Document Control')   
ORDER BY Department, Rate DESC;  

Department             LastName               Rate                  CumeDist               PctRank  
---------------------- ---------------------- --------------------- ---------------------- ----------------------  
Document Control       Arifin                 17.7885               1                      1  
Document Control       Norred                 16.8269               0.8                    0.5  
Document Control       Kharatishvili          16.8269               0.8                    0.5  
Document Control       Chai                   10.25                 0.4                    0  
Document Control       Berge                  10.25                 0.4                    0  
Information Services   Trenary                50.4808               1                      1  
Information Services   Conroy                 39.6635               0.9                    0.888888888888889  
Information Services   Ajenstat               38.4615               0.8                    0.666666666666667  
Information Services   Wilson                 38.4615               0.8                    0.666666666666667  
Information Services   Sharma                 32.4519               0.6                    0.444444444444444  
Information Services   Connelly               32.4519               0.6                    0.444444444444444  
Information Services   Berg                   27.4038               0.4                    0  
Information Services   Meyyappan              27.4038               0.4                    0  
Information Services   Bacon                  27.4038               0.4                    0  
Information Services   Bueno                  27.4038               0.4                    0  
(15 row(s) affected)