The inverse distribution analytical functions return a k-th percentile value, which can be used to help establish a threshold acceptance value for a set of data.
Similarly, the inverse distribution functions require a WITHIN GROUP (ORDER BY) clause. The ORDER BY specifies the expression on which the percentile function is performed and the order in which the rows are sorted in each group. This ORDER BY clause is used only within the WITHIN GROUP clause and is not an ORDER BY for the SELECT. The WITHIN GROUP clause distributes the query result into an ordered data set from which the function calculates a result.