DO IF condition.
...
[ELSE IF condition.
...
]...
[ELSE.
...]
END IF.
DO IF allows one of several sets of transformations to be
executed, depending on user-specified conditions.
If the specified boolean expression evaluates as true, then the block
of code following DO IF is executed. If it evaluates as
missing, then
none of the code blocks is executed. If it is false, then
the boolean expression on the first ELSE IF, if present, is tested in
turn, with the same rules applied. If all expressions evaluate to
false, then the ELSE code block is executed, if it is present.
When DO IF or ELSE IF is specified following TEMPORARY
(see TEMPORARY), the LAG function may not be used
(see LAG).