BY-Group Processing in the DATA Step

Definitions for BY-Group Processing

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. 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.
BY variable: names a variable or variables by which the data set is sorted or indexed. All data sets must be ordered or indexed on the values of the BY variable if you use the SET, MERGE, or UPDATE statements. If you use MODIFY, data does not need to be ordered. However, your program might run more efficiently with ordered data. All data sets that are being combined must include one or more BY variables. The position of the BY variable in the observations does not matter.
BY value: is the value or formatted value of the BY variable.
BY group: 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.
FIRST.variable and LAST.variable: 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. For more information, see How the DATA Step Identifies BY Groups.

For more information about BY-Group processing, see Reading, Combining, and Modifying SAS Data Sets. See also Combining and Modifying SAS Data Sets: Examples.

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