Trinomial function in NVL oracle

NVL: if the first parameter is empty use the value of the second parameter NVL(,)

NVL2: If the first parameter is empty, take the second parameter value, if the first parameter is not empty, take the third parameter value

 

SELECT COUNT(*) count,

       (SELECT NVL(MAX(SET_USER_NUM), 999999)

          FROM TB_CUS_FIRM T1

         WHERE CUSTOMER_KEY = 545) SET_USER_NUM

  FROM TB_CUS_USER

 WHERE CUSTOMER_KEY =545 AND USER_LEVEL = 1

 GROUP BY CUSTOMER_KEY