You can combine or modify WHERE expressions by using
the logical operators (also called Boolean operators) AND, OR, and
NOT. The basic syntax of a compound WHERE expression is as follows:

WHERE where-expression-1 AND | OR | NOT where-expression-n

AND combines two conditions
by finding observations that satisfy both conditions. For example:

where skill eq 'java' and years eq 4;

OR combines two conditions
by finding observations that satisfy either condition or both. For
example:

where skill eq 'java' or years eq 4;

Even though SAS evaluates logical operators in a specific order,
you can control the order of evaluation by nesting expressions in
parentheses. That is, an expression enclosed in parentheses is processed
before one not enclosed. The expression within the innermost set of
parentheses is processed first, followed by the next deepest, moving
outward until all parentheses have been processed.

For example, suppose
you want a list of all the Canadian sites that have both SAS/GRAPH and SAS/STAT software, so you issue the following expression:

where product='GRAPH' or product='STAT' and country='Canada';

The result, however,
includes all sites that license SAS/GRAPH software along with the
Canadian sites that license SAS/STAT software. To obtain the correct
results, you can use parentheses, which causes SAS to evaluate the
comparisons within the parentheses first, providing a list of sites
with either product licenses, then the result is used for the remaining condition:

where (product='GRAPH' or product='STAT') and country='Canada';