SQL-在窗口函数内过滤

Question

I'm using SQL Server 2014

I am trying to remove some noise in a dataset by taking the average of all values in the 90th percentile of a group. Here is the query:

SELECT
    DISTINCT EventLocation,
    PERCENTILE_CONT(.90) 
        WITHIN GROUP (ORDER BY (DATEDIFF(MINUTE, StartTime, EndTime)) ASC) 
        OVER (PARTITION BY EventLocation) 
        AS 'P90',
    AVG(DATEDIFF(MINUTE, StartTime, EndTime))
        OVER (PARTITION BY EventLocation) 
        AS 'Mean'
  FROM MyTable
  ORDER BY N DESC

Currently there are 2 calculated columns:

• The 90th percentile value (of PARTITION population)
• The mean (of PARTITION population)

I Want to add another column for:

• The mean of values (in a PARTITION population) <= the 90th percentile value (of that PARTITION population)

Something like:

AVG(DATEDIFF(MINUTE, StartTime, EndTime))
    OVER (PARTITION BY EventLocation) 
    HAVING (DATEDIFF(MINUTE, StartTime, EndTime) <= [ 90th percentile value ])
    AS 'Mean90'

I'm not exactly sure how to approach this since it is referencing the 90th percentile value that was just deifned in P90 ...maybe a user-defined function applied group-wise, creating multiple tables and joining them, or something else.

Answer 1

As Gordon said, a CTE is a common way to solve a problem like this. Store the results of your original query in the CTE, then select the content of the CTE and add the work you want done using the column aliases you defined.

;WITH IntermediateResults AS (
    SELECT
        DISTINCT EventLocation,
        PERCENTILE_CONT(.90) 
            WITHIN GROUP (ORDER BY (DATEDIFF(MINUTE, StartTime, EndTime)) ASC) 
            OVER (PARTITION BY EventLocation) 
            AS 'P90',
        AVG(DATEDIFF(MINUTE, StartTime, EndTime))
            OVER (PARTITION BY EventLocation) 
            AS 'Mean'
    FROM MyTable
    ORDER BY N DESC
)

SELECT
    *,
    AVG(DATEDIFF(MINUTE, StartTime, EndTime))
        OVER (PARTITION BY EventLocation) 
        HAVING (DATEDIFF(MINUTE, StartTime, EndTime) <= P90)
        AS 'Mean90'
FROM IntermediateResults