DD's SAS notes 28Aug2012-经管之家官网！

28Aug2012:
Special SAS Data Sets:

Null Data Sets

If you want to execute a DATA step but do not want to create a SAS data set, you can specify the keyword _NULL_ as the data set name. The following statement begins a DATA step that does not create a data set:
data _null_;
Using _NULL_ causes SAS to execute the DATA step as if it were creating a new data set, but no observations or variables are written to an output data set. This process can be a more efficient use of computer resources if you are using the DATA step for some function, such as report writing, for which the output of the DATA step does not need to be stored as a SAS data set.

Default Data Sets

SAS keeps track of the most recently created SAS data set through the reserved name _LAST_. When you execute a DATA or PROC step without specifying an input data set, by default, SAS uses the _LAST_ data set. Some functions use the _LAST_ default as well.
The _LAST_= system option enables you to designate a data set as the _LAST_ data set. The name you specify is used as the default data set until you create a new data set. You can use the _LAST_= system option when you want to use an existing permanent data set for a SAS job that contains a number of procedure steps. Issuing the _LAST_= system option enables you to avoid specifying the SAS data set name in each procedure statement. The following OPTIONS statement specifies a default SAS data set:
options _last_=schedule.january;

Automatic Naming Convention

If you do not specify a SAS data set name or the reserved name _NULL_ in a DATA statement, SAS automatically creates data sets with the names DATA1, DATA2, and so on, to successive data sets in the WORK or USER library. This feature is referred to as the DATAn naming convention. The following statement produces a SAS data set using the DATAn naming convention:
data;