The Basics of Match-Merge Processing
The match-merging examples in this book are straightforward. However, match-merging can be more complex, depending on your data and on the output data set that you want to create. To predict the results of match-merges correctly, you need to understand how the DATA step performs match-merges.
When you submit a DATA step, it is processed in two phases:
the compilation phase, in which SAS checks the syntax of the SAS statements and compiles them (translates them into machine code). During this phase, SAS also sets up descriptor information for the output data set and creates the PDV.
the execution phase in which the DATA step reads data and executes any subsequent programming ...