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
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.
DO IF or
ELSE IF is specified following
(see TEMPORARY), the
LAG function may not be used