What is difference between IIF and DECODE function

You can use nested IIF statements to test multiple conditions. The following example tests for various conditions and returns 0 if sales is zero or negative: 
IIF( SALES > 0, IIF( SALES < 50, SALARY1, IIF( SALES < 100, SALARY2, IIF( SALES < 200, SALARY3, BONUS))), 0 )
You can use DECODE instead of IIF in many cases. DECODE may improve readability. The following
shows how you can use DECODE instead of IIF :
SALES > 0 and SALES < 50, SALARY1,
SALES > 49 AND SALES < 100, SALARY2,
SALES > 99 AND SALES < 200, SALARY3,
SALES > 199, BONUS)