查询以从单个查询获取多个结果或返回null

Question

I have a Holiday table with following fields

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

and the sample data is

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

I have used the following query to get the holiday_date of a particular month/year: (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

This works fine if the result is null(ie for month/year= JUN/2016) but gives "single-row subquery returns more than one row" error when the month contains more than one holiday (ie for month/year= OCT/2016)

How do I write a query such that if the query returns multiple rows then display those results else display '0' for NULL result?

[ NOTE : The holidays are as per the Indian calendar]

Answer 1

You should consider changing your query to this:

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

You need to apply the nvl function to the column, to to the result, like this:

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%'

On a side note: learn how to write sargable queries

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'))