查詢以從單個查詢獲取多個結果或返回null

Question

我有一個假期表，其中包含以下字段

COLUMN_NAME         DATA_TYPE
-------------------------------
HOLIDAY_ID          NUMBER(10,0)
HOLIDAY_NAME        VARCHAR2(150 BYTE)
HOLIDAY_DATE        TIMESTAMP(6)
DAY_OF_WEEK         VARCHAR2(20 BYTE)
HOLIDAY_TYPE_ID     NUMBER

樣本數據是

HOLIDAY_ID   HOLIDAY_NAME     HOLIDAY_DATE                      DAY_OF_WEEK     HOLIDAY_TYPE_ID
-------------------------------------------------------------------------------
305         Republic Day      26-01-17 12:00:00.000000000 AM    Thursday         1

306         Maha Shivratri    24-02-17 12:00:00.000000000 AM     Friday          1

我使用以下查詢獲取特定月份/年份的holiday_date： （QUERY）

select nvl((select to_char(holiday_date,'DD/MM/YY') holiday 
from admin_holiday where to_char(holiday_date,'DD-MON-YYYY') like '%-JUN-2016%' 
and holiday_type_id=1 and to_char(holiday_date,'Day') not like 'Sun%' 
and to_char(holiday_date,'Day') not like 'Sat%'),'0') holiday from dual

如果結果為空（即，月/年= JUN / 2016），但工作正常，但當月份包含多個假期（例如，月/年= OCT /）時，出現“單行子查詢返回多個行”錯誤2016）

如何編寫查詢，以便如果查詢返回多行然后顯示那些結果，否則顯示NULL結果為'0'？

[ 注意 ：假期按照印度日歷]

Answer 1

您應該考慮將查詢更改為此：

SELECT NVL(TO_CHAR(holiday_date,'DD/MM/YY'), '0') holiday
FROM admin_holiday 
WHERE TO_CHAR(holiday_date,'MON-YYYY') = 'JUN-2016' 
--WHERE TO_CHAR(holiday_date,'yyyymm') = '201606'
    AND holiday_type_id = 1 
    AND TO_CHAR(holiday_date,'Day') NOT IN ('Sun', 'Sat');

Answer 2

您需要將nvl函數應用於列和結果，如下所示：

select nvl(to_char(holiday_date,'DD/MM/YY'),'0') holiday 
from (select 1 as dummy from dual) d left join admin_holiday 
on to_char(holiday_date,'DD-MON-YYYY') like '%-JUN-2016%' 
and holiday_type_id=1 and to_char(holiday_date,'Day') not like 'Sun%' 
and to_char(holiday_date,'Day') not like 'Sat%'

附帶說明：了解如何編寫可查詢的查詢

Answer 3

SELECT NVL(TO_CHAR(holiday_date,'DD/MM/YY'), '0') holiday
FROM admin_holiday 
WHERE TO_CHAR(holiday_date,'MON-YYYY') = 'JUN-2016' 
--WHERE TO_CHAR(holiday_date,'yyyymm') = '201606'
    AND holiday_type_id = 1 
    AND TO_CHAR(holiday_date,'Day') NOT IN ('Sun', 'Sat')
union all
select '0' from dual where not exists(SELECT NVL(TO_CHAR(holiday_date,'DD/MM/YY'), '0') holiday
FROM admin_holiday 
WHERE TO_CHAR(holiday_date,'MON-YYYY') = 'JUN-2016' 
--WHERE TO_CHAR(holiday_date,'yyyymm') = '201606'
    AND holiday_type_id = 1 
    AND TO_CHAR(holiday_date,'Day') NOT IN ('Sun', 'Sat'))