NVL:如果第一个参数为空用第二个参数的值NVL（,）
NVL2:如果第一个参数为空取第二个参数值，如果第一个参数不为空，取第三个参数值
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