BY-group processing 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. You can use BY-group processing in both
DATA and PROC steps.
The most common use of BY-group
processing in the DATA step is to combine two or more SAS data sets
by using the BY statement with a SET, MERGE, MODIFY, or UPDATE statement.
When you create reports or summaries with SAS procedures, BY-group
processing enables you to group information in the output according
to values of one or more variables.