SQL Server ->> PERCENTILE_CONT、PERCENTILE_DISC 和 PERCENT_RANK 函数

PERCENTILE_CONT和PERCENTILE_DISC都是为了计算百分位的数值，比如计算在某个百分位时某个栏位的数值是多少。他们的区别就是前者是连续型，后者是离散型。CONT代表continuous，DISC代表discrete。PERCENTILE_CONT是连续型意味它考虑的是区间，所以值是绝对的中间值。而PERCENTILE_DISC是离散型，所以它更多考虑向上或者向下取舍，而不会考虑区间。下面是MSDN的一个例子：

USE AdventureWorks2012;  

SELECT DISTINCT Name AS DepartmentName
      ,PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY ph.Rate)
                            OVER (PARTITION BY Name) AS MedianCont
      ,PERCENTILE_DISC(0.5) WITHIN GROUP (ORDER BY ph.Rate)
                            OVER (PARTITION BY Name) AS MedianDisc
FROM HumanResources.Department AS d
INNER JOIN HumanResources.EmployeeDepartmentHistory AS dh
    ON dh.DepartmentID = d.DepartmentID
INNER JOIN HumanResources.EmployeePayHistory AS ph
    ON ph.BusinessEntityID = dh.BusinessEntityID
WHERE dh.EndDate IS NULL;  

PERCENT_RANK虽然也带有PERCENT，但是和前两个没有什么关系。它是为了计算当前值按百分比计算所处在百分位位置。

参考：

PERCENTILE_CONT (Transact-SQL)

PERCENTILE_DISC (Transact-SQL)

PERCENT_RANK (Transact-SQL)