Need query to return zero when count(*) doesn't match any records

Question

SELECT COUNT(*),TRUNC(TEST_DATE_CREATE) 
FROM TEST_TABLE 
WHERE TEST_DATE_CREATE > (SYSDATE - 10)
GROUP BY TRUNC(TEST_DATE_CREATE);

I need the above query to return the count 0 in case of no records present for a particular date. This doesn't seem to be happening.

I tried NVL, decode. Found that the issue is due to group by clause, but not sure how to fix this.

Please help !!

Answer 1

You need to generate a list of the dates you're interested in, and then do a left outer join to your table to find records that match each date. Something like this:

with tmp_dates as (
    select trunc(sysdate) - level + 1 as tmp_date
    from dual
    connect by level <= 10
)
select count(tt.test_date_create), td.tmp_date
from tmp_dates td
left join test_table tt on trunc(tt.test_date_create) = td.tmp_date
group by td.tmp_date
order by tmp_date;

The common table expression on its own generates a list of dates:

select trunc(sysdate) - level + 1 as tmp_date
from dual
connect by level <= 10;

TMP_DATE
---------
10-JUN-13 
09-JUN-13 
08-JUN-13 
07-JUN-13 
06-JUN-13 
05-JUN-13 
04-JUN-13 
03-JUN-13 
02-JUN-13 
01-JUN-13 

You can adjust the level limit and how it's added to sysdate to modify the range, e.g. to look further back or to exclude today.

The main select then uses that to look for matching records. Because it is an outer join it lists all the generated dates, and a count of zero where there are no matches.

SQL Fiddle.

Answer 2

Hard to understand what you want, but

select decode(count(*), 0, null, count(*)) 

will return null when there are no records in the group.

UPDATE: Now is clearer what you want:

select date_code, cnt
from(
    select trunc(sysdate-10) + level - 1 as date_code
    from dual
    connect by level <= 10
    )
left join(
    SELECT COUNT(*) cnt,TRUNC(TEST_DATE_CREATE)  trunc_create
    FROM TEST_TABLE 
    WHERE TEST_DATE_CREATE > (SYSDATE - 10)
    GROUP BY TRUNC(TEST_DATE_CREATE);
) on trunc_create = date_code

UPDATE2: Now is clearer what you want, zeroes not nulls:

select date_code, count(*) 
from(
    select trunc(sysdate-10) + level - 1 as date_code
    from dual
    connect by level <= 10
    )
left join TEST_TABLE on TRUNC(TEST_DATE_CREATE) = date_code
GROUP BY date_code;