SAS can access your data either sequentially or directly. When writing a SAS DATA step, your code might enable you to specify which of the two access methods SAS should use. PROC SQL decides for you which method is more efficient to use based on the code and data sets that you have specified.
Sequential access means that SAS accesses the observations in your data set in the order in which they appear in the physical file.
Direct access means that SAS goes straight to an observation in a SAS data set without having to process each observation that precedes it.
Computer resources such as CPU time, I/O, and disk storage can be conserved based on the access method that you choose.