if statement selects one of two statements for execution,
depending on a condition.
IF expr THEN statement1 [ ELSE statement2 ]
The condition expression
expr is evaluated. Its value must
be a scalar. If the value is non-zero, then statement
statement1 is executed. If the condition evaluates to
zero, and the optional
else part was specified, then
statement2 is executed. If the condition evaluates to zero
else part was specified, then this statement does nothing
and execution passes on directly to the next statement.