Dataset File-Handling statements - SET/MERGE/UPDATE/MODIFY

Totally there are 4 dataset file-handling statements.
All can support the fomula as follows:

DATA DS;
SET DS1 DS2; *SET can be replaced with MERGE/UPDATE/MODIFY;
BY VAR;
RUN;

However, they have different meaning.

SET statement: interleave two or more SAS datasets. We can get an ordered dataset without PROC SORT.
Merge statement: match-merging, which is very popular.
Update statement: update Master dataset with Transaction one, which is the only formula. Before using it, please read carefully the requirements in manual.
Modify statement: update Master dataset in place using matching access method. Although it is a powerful tool, many programmer would like to bypass it using very complicated code.