實現功能：
1.先查詢status=2的記錄，如果查詢到記錄則返回第一條記錄的Product_Name；
2.如果查詢不到status=2的記錄，則判斷status=1的記錄是否存在，不存在則返回“請耐心等待!”，存在則返回“拆煙完畢!”
實現思路：
1、使用case語句返回不同的內容：
case語句格式：
case when 判斷語句1 then 返回1 when 判斷語句2 then 返回2 …… else 返回n end
2、解決無記錄時需要返回一條null的數據：
select Product_name from v_supply_sj union select null from dual
3、解決有記錄的情況下還會附帶一條null數據的問題：
通過判斷初始查詢結果count（*）是否等於0來決定返回的記錄數
最終查詢語句：
select case when t1.Product_name is not null then t1.product_name when t2.Product_name is not null then '拆煙完畢' else '請耐心等待' end as ledtext from (select rownum as q, Product_name from (select Product_name from( select rownum as p,Product_name from v_supply_sj where to_addr = 'SJ03' and bill_type=5 and status = 2 and up_down = '1' ) where rownum<=1 union select null from dual ) where rownum<= case when (select count(*) from (select Product_name from( select rownum as p,Product_name from v_supply_sj where to_addr = 'SJ03' and bill_type=5 and status = 2 and up_down = '1' ) where rownum<=1 union select null from dual ))=1 then 1 else (select count(*) from (select Product_name from( select rownum as p,Product_name from v_supply_sj where to_addr = 'SJ03' and bill_type=5 and status = 2 and up_down = '1' ) where rownum<=1 ) ) end ) t1 left join (select rownum as q, Product_name from (select Product_name from( select rownum as p,Product_name from v_supply_sj where to_addr = 'SJ03' and bill_type=5 and status = 1 and up_down = '1' ) where rownum<=1 union select null from dual ) where rownum<= case when (select count(*) from (select Product_name from( select rownum as p,Product_name from v_supply_sj where to_addr = 'SJ03' and bill_type=5 and status = 1 and up_down = '1' ) where rownum<=1 union select null from dual ))=1 then 1 else (select count(*) from (select Product_name from( select rownum as p,Product_name from v_supply_sj where to_addr = 'SJ03' and bill_type=5 and status = 1 and up_down = '1' ) where rownum<=1 ) ) end ) t2 on t1.q=t2.q