Rounding to Previous 2 hour Window Oracle SQL - sql

Question

I'm trying to round a date (date datatype which includes timestamp too) to the previous 2 hour block. e.g 13:23 -> 12:00, 18:12 -> 18:00
I had it working in MySQL using a MOD function as:
DATE_ADD(DATE(DATE_ADD(created_at, INTERVAL - 7 HOUR)), INTERVAL HOUR(DATE_ADD(created_at, INTERVAL - 6 HOUR)) - MOD(HOUR(DATE_ADD(created_at, INTERVAL - 6 HOUR)), 2) HOUR) AS Window_Start
**Complexity added as I'm also shifting the time a) to correct for a 7 hr time zone difference and b) because I need to offset the time by 1 hr before grouping it. But that's not where the issue is.
But I can't get it to work on an Oracle platform. Specifically, I can't seem to extract the hour of the time as a number which to feed into the MOD(). I've been trying with CAST, and TO_TIMESTAMP and TO_CHAR but nothing seems to work. The usual error is "inconsistent datatypes".
EXTRACT only works with with timestamp type, not date. And TO_TIMESTAMP only works on strings. But EXTRACT(TO_TIMESTAMP(TO_CHAR( doesn't work either.
I'm sure there's an easier way to do this...

Answer 1

In Oracle, one method is using date arithmetic. For example:
select date '2000-01-01' + floor((sysdate - date '2000-01-01') * (24 / 2)) / (24 / 2)
from dual;
The "24" is for hours in a day. The "2" is for the two hour period you want to truncate to.

Answer 2

I'm not sure that this is the answer but it'll give you a rabbit hole to chase:
select sysdate as curr_dt, to_date(to_char(sysdate - interval '2' hour, 'yyyy-mm-dd HH24')) as two_hours_ago from dual;
Hope it helps!

Answer 3

To get the hour in a number format this will work.
select TO_NUMBER(TO_CHAR(SYSDATE,'HH')) from dual
You can select each piece individually using the time stamp portion of this date format DD/MM/YYYY HH:MI:SS.

Answer 4

Some time ago I wrote this generic function:
CREATE FUNCTION MakeInterval(ts IN TIMESTAMP, roundInterval IN INTERVAL DAY TO SECOND) RETURN TIMESTAMP DETERMINISTIC IS
denom INTEGER;
BEGIN
IF roundInterval >= INTERVAL '1' HOUR THEN
denom := EXTRACT(HOUR FROM roundInterval);
IF MOD(24, denom) <> 0 THEN
RAISE VALUE_ERROR;
END IF;
RETURN TRUNC(ts) + TRUNC(EXTRACT(HOUR FROM ts) / denom) * denom * INTERVAL '1' HOUR;
ELSIF roundInterval >= INTERVAL '1' MINUTE THEN
denom := EXTRACT(MINUTE FROM roundInterval);
IF MOD(60, denom) <> 0 THEN
RAISE VALUE_ERROR;
END IF;
RETURN TRUNC(ts, 'hh') + TRUNC(EXTRACT(MINUTE FROM ts) / denom) * denom * INTERVAL '1' MINUTE;
ELSE
denom := EXTRACT(SECOND FROM roundInterval);
IF MOD(60, denom) <> 0 THEN
RAISE VALUE_ERROR;
END IF;
RETURN TRUNC(ts, 'mi') + TRUNC(EXTRACT(SECOND FROM ts) / denom) * denom * INTERVAL '1' SECOND;
END IF;
END MakeInterval;
In your case the usage would be
SELECT MakeInterval(created_at, INTERVAL '2' HOUR)
FROM ...
But perhaps this would be an overkill in your situation, usage without the function would be:
SELECT TRUNC(created_at) + TRUNC(EXTRACT(HOUR FROM created_at) / 2) * 2 * INTERVAL '1' HOUR
FROM ...