Question-: What is the use of _NULL_ in SAS?
Answer: In SAS _NULL_ is a reserved keyword which specifies a SAS data set that has no observations and no variables. For example, if you specify _NULL_ as an output dataset then it would not no output generated. The _NULL_ data set is often used when you want to execute DATA step code that displays a result, defines a macro variable, writes a text file, or makes calls to the EXECUTE subroutine. In those cases, you are interested in the "side effect" of the DATA step and rarely want to write a data set to disk.
Question-: What do you mean by SAS expression?
Answer: SAS expression represent a single value that is used in the program, and it can be a constant, variable, or an array element.
Question-: What is the difference between NODUPKEY and NODUP?
Answer: Please find the difference - NONDUP: In this case you specify on which variable you want to sort the data, once data is sorted it removes the contiguous duplicate records. - NONDUPKEY: As name suggest it first creates the keys in the data and then remove the observations which are having duplicate keys. You specify which all variable should be in the key using the BY statement
Question-: What is an alternate way to remove duplicate data?
Answer: You can use the FIRST and LAST expressions to remove the duplicate data. Variable created when sorting containing Boolean for each BY variable. - : It means current observation is either ‘FIRST/LAST’ in the By group. - : It means current observation is not either ‘FIRST/LAST’ in the By group.