Counting records and grouping them by the hour - sql

Question

I'm trying to count the records in my table and grouping them by hour, i'm getting results with my query but I want it to return every hour even if there are no records.
My current query is,
SELECT nvl(count(*),0) AS transactioncount, trunc(date_modified, 'HH') as TRANSACTIONDATE
FROM TABLE
WHERE date_modified between to_date('23-JAN-19 07:00:00','dd-MON-yy hh24:mi:ss') and to_date('24-Jan-19 06:59:59','dd-MON-yy hh24:mi:ss')
group by trunc(date_modified, 'HH');
This returns a result like this,
TRANSACTIONCOUNT | TRANSACTIONDATE
43 | 23-Jan-19 07:00:00
47 | 23-Jan-19 08:00:00
156 | 23-Jan-19 14:00:00
558 | 23-Jan-19 15:00:00
What I want is for it to return every hour between my 2 dates so,
TRANSACTIONCOUNT | TRANSACTIONDATE
43 | 23-Jan-19 07:00:00
47 | 23-Jan-19 08:00:00
0 | 23-Jan-19 09:00:00
0 | 23-Jan-19 10:00:00
0 | 23-Jan-19 11:00:00
0 | 23-Jan-19 12:00:00
0 | 23-Jan-19 13:00:00
156 | 23-Jan-19 14:00:00
558 | 23-Jan-19 15:00:00
--......
0 | 24-Jan-19 00:00:00
0 | 24-Jan-19 01:00:00
0 | 24-Jan-19 02:00:00
--and so on

Answer 1

To fill the holes in the transaction hours you create first a complete table of hours.
You may use Recursive Subquery Factoring to do it
WITH hour_table(TRANSACTIONDATE) AS (
SELECT to_date('23-JAN-19 07:00:00','dd-MON-yy hh24:mi:ss') /* init hour here */
FROM DUAL
UNION ALL
SELECT TRANSACTIONDATE + 1/24
FROM hour_table
WHERE TRANSACTIONDATE + 1/24 < to_date('24-JAN-19 06:59:59','dd-MON-yy hh24:mi:ss') /* limit here */
)
select * from hour_table;
TRANSACTIONDATE
-------------------
23.01.2019 07:00:00
23.01.2019 08:00:00
...
24.01.2019 05:00:00
24.01.2019 06:00:00
Note that you use the staring and ending date in this query, the starting date must be exact an hour.
Next step is as simple as to outer join this hour table to your aggregation and set the default value for the missing hours with NVL.
with hour_table(TRANSACTIONDATE) AS (
SELECT to_date('23-JAN-19 07:00:00','dd-MON-yy hh24:mi:ss') /* init hour here */
FROM DUAL
UNION ALL
SELECT TRANSACTIONDATE + 1/24
FROM hour_table
WHERE TRANSACTIONDATE + 1/24 < to_date('24-JAN-19 06:59:59','dd-MON-yy hh24:mi:ss') /* limit */
),
agg as (
SELECT nvl(count(*),0) AS transactioncount, trunc(date_modified, 'HH') as TRANSACTIONDATE
FROM "TABLE"
WHERE date_modified between to_date('23-JAN-19 07:00:00','dd-MON-yy hh24:mi:ss') and to_date('24-Jan-19 06:59:59','dd-MON-yy hh24:mi:ss')
group by trunc(date_modified, 'HH')
)
select t.TRANSACTIONDATE, nvl(transactioncount,0) transactioncount
from hour_table t
left outer join agg a
on t.TRANSACTIONDATE = a.TRANSACTIONDATE
order by 1;

Answer 2

You might consider using the following with CONNECT BY level logic :
SELECT sum(transactioncount) as transactioncount, transactiondate
FROM
(
with "TABLE"(date_modified) as
(
SELECT timestamp'2019-01-23 08:00:00' FROM dual union all
SELECT timestamp'2019-01-23 08:30:00' FROM dual union all
SELECT timestamp'2019-01-23 09:00:00' FROM dual union all
SELECT timestamp'2019-01-24 05:01:00' FROM dual
)
SELECT nvl(count(*),0) AS transactioncount, trunc(date_modified, 'hh24') as transactiondate
FROM "TABLE" t
GROUP BY trunc(date_modified, 'HH24')
UNION ALL
SELECT 0, timestamp'2019-01-23 07:00:00' + ( level - 1 )/24
FROM dual
CONNECT BY level <= 24 * extract( day from
timestamp'2019-01-24 06:59:59'-
timestamp'2019-01-23 07:00:00') +
extract( hour from
timestamp'2019-01-24 06:59:59'-
timestamp'2019-01-23 07:00:00') + 1
)
GROUP BY transactiondate
ORDER BY transactiondate
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