The TopPercent function returns, in order of decreasing rank, the top-most rows of a table whose cumulative total is at least a specified percentage.
TopPercent(<table expression>, <rank expression>, <percent>)
An expression that returns a table, such as a <table column reference>, or a function that returns a table.
The TopPercent function returns the top-most rows in decreasing order of rank based on the evaluated value of the <rank expression> argument for each row, such that the sum of the <rank expression> values is at least the given percentage that is specified by the <percent> argument. TopPercent returns the smallest number of elements possible while still meeting the specified percent value.
The following example creates a prediction query against the Association model that you build by using the Basic Data Mining Tutorial.
To understand how TopPercent works, it might be helpful to first execute a prediction query that returns only the nested table.
SELECT Predict ([Association].[v Assoc Seq Line Items], INCLUDE_STATISTICS, 10) FROM [Association] NATURAL PREDICTION JOIN SELECT (SELECT 'Women''s Mountain Shorts' as [Model]) AS [v Assoc Seq Line Items]) AS t
In this example, the value supplied as input contains a single quotation mark, and therefore must be escaped by prefacing it with another single quotation mark. If you are not sure of the syntax for inserting an escape character, you can use the Prediction Query Builder to create the query. When you select the value from the dropdown list, the required escape character is inserted for you. For more information, see Create a Singleton Query in the Data Mining Designer.
|Mountain Tire Tube||1992||0.133879965||0.125304948|
|Road Tire Tube||1588||0.106727603||0.101229538|
|Fender Set - Mountain||1415||0.095100477||0.090718432|
|Mountain Bottle Cage||1367||0.091874454||0.087780332|
|Road Bottle Cage||1195||0.080314537||0.077173962|
The TopPercent function takes the results of this query and returns the rows with the greatest values that sum to the specified percentage.
SELECT TopPercent ( Predict ([Association].[v Assoc Seq Line Items],INCLUDE_STATISTICS,10), $SUPPORT, 50) FROM [Association] NATURAL PREDICTION JOIN (SELECT (SELECT 'Women''s Mountain Shorts' as [Model]) AS [v Assoc Seq Line Items]) AS t
The first argument to the TopPercent function is the name of a table column. In this example, the nested table is returned by calling the Predict function and using the INCLUDE_STATISTICS argument.
The second argument to the TopPercent function is the column in the nested table that you use to order the results. In this example, the INCLUDE_STATISTICS option returns the columns $SUPPORT, $PROBABILTY, and $ADJUSTED PROBABILITY. This example uses $SUPPORT because support values are not fractional and therefore are easier to verify.
The third argument to the TopPercent function specifies the percentage, as a double. To get the rows for the top products that sum to 50 percent of the total support, you type 50.
|Mountain Tire Tube||1992||0.133…||0.12…|
Note This example is provided only to illustrate the usage of TopPercent. Depending on the size of your data set, this query might take a long time to run.
The MDX functions for TOPPERCENT and BOTTOMPERCENT can generate unexpected results when the values used to calculate the percentage include negative numbers. This behavior does not affect the DMX functions. For more information, see BottomPercent (MDX).