is a method of processing observations from one or more SAS data sets that are grouped or ordered by values of one or more common variables.

names a variable or variables by which the data set is sorted. All data sets must be ordered by the values of the BY variable.

is the value of the BY variable.

includes all observations with the same BY value. If you use more than one variable in a BY statement, a BY group is a group of observations with the same combination of values for these variables. Each BY group has a unique combination of values for the variables.

are variables that SAS creates for each BY variable. SAS sets FIRST.variable when it is processing the first observation in a BY group, and sets LAST.variable when it is processing the last observation in a BY group. These assignments enable you to take different actions, based on whether processing is starting for a new BY group or ending for a BY group.