If the logical expression on DO IF is true, the commands immediately following DO IF are executed up to the next ELSE IF, ELSE, or END IF command. Control
then passes to the first statement following END IF.
If the expression on DO
IF is false, control passes to the following ELSE IF command. Multiple ELSE IF commands are evaluated in the order
in which they are specified until the logical expression on one of
them is true. Commands following that ELSE
IF command are executed up to the ELSE or END IF command,
and control passes to the first statement following END IF.
If none of the expressions are true on the DO IF or any of the ELSE IF commands, the commands following ELSE are executed and control passes out
of the structure. If there is no ELSE command, a case goes through the entire structure with no change.
Missing values returned by the logical expression
on DO IF or on any ELSE IF cause control to pass to the END IF command at that point.