How to group and count by date and 8-hr interval on those dates in datetime SQL column? - sql

Question

I have a table with a column visit_date that is a datetime object with format YYYY-MM-DD HH:MI:SS that looks like the following:
visit_date |visit_id
-------------------|-----
2010-11-01 00:02:00|92314
2010-11-01 23:05:21|23498
2010-11-01 12:42:31|12343
2010-11-02 05:13:21|79881
2010-11-02 14:35:15|22134
2010-11-02 16:12:23|12348
2010-11-03 01:22:44|12384
2010-11-03 05:23:41|12394
2010-11-03 15:13:55|99384
I would like to group by date and by 8-hr window on that date such that I have:
interval |count
-------------------|-----
2010-11-01 00:00:00|1
2010-11-01 08:00:00|2
2010-11-01 16:00:00|3
2010-11-02 00:00:00|4
2010-11-02 08:00:00|5
2010-11-02 16:00:00|6
2010-11-03 00:00:00|7
2010-11-03 08:00:00|8
2010-11-03 16:00:00|9
My original query (using only dates) was:
SELECT CAST(visit_date as DATE), count(1) as count
FROM table
GROUP BY CAST(visit_date as DATE)
ORDER BY CAST(visit_date as DATE)
But that only groups by date.
Is there a recommended way to get interval counts for each interval per day? I have seen implementations using DATEADD and DATEPART but not sure which makes the most sense in this situation.
Thanks!

Answer 1

Add the hours in to what you group and count:
SELECT
CAST(visit_date as DATE),
HOUR(visit_date)/8 as ival8h
count(1) as count
FROM table
GROUP BY CAST(visit_date as DATE), HOUR(visit_date)/8
ORDER BY CAST(visit_date as DATE)
The hour function returns the hour number of the passed date, divide it by 8 to get an int of the interval, so 0 to 7 becomes 0, 8 to 16 becomes 1 etc
If you want it back as a time pegged to a round 8h multiply it by 8 again and format it to NN:00:00, or add it to the date, thus:
SELECT
DATEADD(hour, (HOUR(visit_date)/8)*8, CAST(CAST(visit_date as DATE) as DATETIME) as quantized_date,
count(1) as count
FROM table
GROUP BY DATEADD(hour, (HOUR(visit_date)/8)*8, CAST(CAST(visit_date as DATE) as DATETIME)
ORDER BY CAST(visit_date as DATE)
This basically rounds the hours down to the lesser 8h market and adds that to midnight. Two casts are required (probably) on the date because DATEADD won't add hours to a date, only a datetime but we need the cast to date to peg the tine element to midnight
If you want there to be a date and a 0 count for periods where no events took place, use a numbers table or row generator and create a sequence of dates to left join your real data onto, then count the real data grouped by the fake dates

Answer 2

Use a cross apply to form 4 shift boundary values, then use those in a case expression to genertate the group by values
SELECT
case
when visit_date >= s1 and visit_date < s2 then s1
when visit_date >= s2 and visit_date < s3 then s2
when visit_date >= s3 and visit_date < s4 then s3
end as shift
, count(1) as count
FROM mytable
CROSS APPLY (
select
cast(CAST(visit_date as DATE)as datetime) s1
, dateadd(hh,8,cast(CAST(visit_date as DATE)as datetime)) s2
, dateadd(hh,16,cast(CAST(visit_date as DATE)as datetime)) s3
, dateadd(hh,24,cast(CAST(visit_date as DATE)as datetime)) s4
) ca
GROUP BY
case
when visit_date >= s1 and visit_date < s2 then s1
when visit_date >= s2 and visit_date < s3 then s2
when visit_date >= s3 and visit_date < s4 then s3
end
ORDER BY shift
result:
+----+---------------------+-------+
| | shift | count |
+----+---------------------+-------+
| 1 | 01.11.2010 00:00:00 | 1 |
| 2 | 01.11.2010 08:00:00 | 1 |
| 3 | 01.11.2010 16:00:00 | 1 |
| 4 | 02.11.2010 00:00:00 | 1 |
| 5 | 02.11.2010 08:00:00 | 1 |
| 6 | 02.11.2010 16:00:00 | 1 |
| 7 | 03.11.2010 00:00:00 | 2 |
| 8 | 03.11.2010 08:00:00 | 1 |
+----+---------------------+-------+

Answer 3

I think the canonical way in SQL Server is to use dateadd() and datediff():
select dateadd(hour, 0, 3 * (datediff(hour, 0, visit_date) / 3)) as day_hour8,
count(*)
from t
group by dateadd(hour, 0, 3 * (datediff(hour, 0, visit_date) / 3))
order by day_hour8;