Most numeric operators yield system-missing when given any system-missing operand. A string operator given any system-missing operand typically results in the empty string. Exceptions are listed under particular operator descriptions.
String user-missing values are not treated specially in expressions.
User-missing values for numeric variables are always transformed into
the system-missing value, except inside the arguments to the
VALUE and SYSMIS functions.
The missing-value functions can be used to precisely control how missing values are treated in expressions. See Missing-Value Functions, for more details.