If/Then

If condition
Then
   ... executable statements ...
Else
   ... executable statements ...
End If;
If condition
Then
   ... executable statements ...
    ElsIf  condition 
    Then
        ... executable statements ...
    Else
        ... executable statements ...
Else
   ... executable statements ...
End If;
    If A = B Then X = 10 
 ElsIf A = C Then X = 20     /* Consider using a */
 ElsIf A = D Then X = 30     /* case statement   */
End If;

---

Nested Ifs

If condition
Then
       If condition
       Then
             ... statements ...
       Else
              If condition
              Then
                    ... statements ...
              ElsIf  condition
                    Then
                         ... statements ...
              End If;
       End If;
End If;

An advantage of these structures is that the inner statements are evaluated only when true. Defer costly execution to inner If statements. Assume determining if C is equal to D is time consuming:

Don't code:
If A=B and C=D Then...
Code:
If A=B
   Then If C = D
        Then...

Note: Null is neither True nor False.