How can write the below
SELECT
    a1.state, b2.name,........* 
FROM
    Table_Name
WHERE
    condition1
    AND condition2
    AND condition3
    AND condition4
    AND IF b2.name = 'mohan' THEN condition 5
How can write the below
SELECT
    a1.state, b2.name,........* 
FROM
    Table_Name
WHERE
    condition1
    AND condition2
    AND condition3
    AND condition4
    AND IF b2.name = 'mohan' THEN condition 5
 
    
    You don't need if.  Just use boolean logic:
where condition1 AND
      condition2 AND
      condition3 AND
      condition4 AND
      (b2.name <> 'mohan' OR condition 5)
