
Sampling 
An aggregate step that performs the sampling of data. 

Statistics 
An aggregate step that operates on grouped rows and computes a new function value for the row given the function specification. This is similar to aggregates in that it operates on grouped rows and a filtering QUALIFY clause analogous to the HAVING clause. It is different from aggregates because it returns a value for each participating row (unless filtered out), and it allows other descriptive columns that are not in the groupby list to be specified in the SELECT list. 

Sum 
An aggregate operator that returns a column value. The value is the arithmetic sum for a specified expression for a group. An expression is a constant or column expression for which the sum value is computed and that cannot contain any ordered analytic or aggregate functions. 