Best practices IF ELSE

When we need to write conditional logic that has several mutually exclusive clauses ;)
(in other words, if one clause is TRUE, no other clause evaluates to TRUE) or
(only one condition to be evaluated at a given point of time)
use the ELSIF construct: , never try to use IF ENDIF construct

suppoose ur intention is to write something like this :

BEGIN
   IF count  = 1
   THEN
      process_A;
   END IF;
   IF count  = 2
   THEN
      process_B;
   END IF;
   ...
   IF count  = 3
   THEN
      process_C;
   END IF;
END;

in the above case
Every IF statement is executed and each condition evaluated.
You should rewrite such logic as follows

BEGIN
   IF count = 1
   THEN
      process_A;
   ELSIF count  = 2
   THEN
      process_B;
   ...
   ELSIF count = 3
   THEN
      process_C;
   END IF;
END;

what is the benefit of these type of code

This structure clearly expresses the underlying "reality"
of your business logic:
if one condition is TRUE, no others can be TRUE.

ELSIF offers the most efficient implementation for
processing mutually exclusive clauses. When one
clause evaluates to TRUE, all subsequent clauses are ignored.

May be in the real world scenario we need to deal with
some convoluted logical expressions like following:

IF condA AND NOT ( condB OR condC ) 
THEN 
   process_A; 
ELSIF condA AND (condB OR condC) 
THEN 
   process_B; 
ELSIF NOT condA AND condD 
THEN
   process_C;
END IF;

from the above example u can very well see that
condition B & C are repeated in two of these conditional statement

It's also fairly common to get a headache trying to make sense
of the code we have written. You can often reduce the trauma
of headache by trading off the simplicity of the( code )
IF statement itself.
Rather than having seperate level for each condtion
we can club the clauses within multiple levels:

we will see ,how to re-write it :

IF condA 
THEN 
   IF (condB OR condC) 
   THEN 
      process_B; 
   ELSE 
      process_A;
   END IF; 
ELSIF condD 
THEN 
   process_C;
END IF; 

re-writing the code what we acheive

"if and when the logic changes, you can change one IF
clause without affecting the logic of others."

Always make your code easier to read and maintain. ;)
( may be for that only we are paid )

may b , now u may start to ask the question
how to write the code which is maintainable ?
Breaking an expression into smaller pieces can aid maintainability;