Oracle working with CONNECT BY and LEVEL with INTERVAL - sql

Question

Alex Poole posted a brilliant solution to another user's issue at Oracle formatting date intervals but
Didn't make it clear earlier that while I have questions on what parts of Alex' original query do, the deisred goal is a result set showing roster entries by individuals in 14 day intervals. Understanding exactly what all parts of the query do is key to always to getting a good result. So much emphasis was placed on certain parts of the query and not enough on where I wanted to arrive. :)
In this part of the query, what does the divisor of 48 have to do with 30 minute intervals? Jokingly, it could be the new Secret of the Universe (Hitchhiker's Guide to the Galaxy) -- it's not 42 anymore but 48. :)
A co worker and I figured it might be for a 6-day work week (48 hours -- 6 8 hour days). Here is Alex' query, note that the object tbl_stat is located at the top of the post, not part of Alex' query per se, it was part of the poster's original question:
with tmp_tab as (
select start_time + (level - 1)/48 as period_start,
start_time + level/48 - interval '1' second as period_end
from (
select to_date(:start_time, 'DD/MM/YYYY HH24:MI:SS') start_time,
to_date(:end_time, 'DD/MM/YYYY HH24:MI:SS') end_time
from dual
)
connect by start_time + (level - 1)/48 < end_time
)
select to_char(tt.period_start, 'DD/MM/YYYY HH24:MI') dt,
count(ts.track_datetime)
from tmp_tab tt
left join tbl_stat ts
on ts.track_datetime between tt.period_start and tt.period_end
group by tt.period_start
order by tt.period_start;
Tha'ts the whole query, but what does the interval " / 48" pertain to as far as setting up 30 minute intervals, etc:
select start_time + (level - 1)/48 as period_start,
start_time + level/48 - interval '1' second as period_end
Thanks, hope that's not too inane a question but I really don't see what's what with it.

Answer 1

The documentation on datetime/interval arithmetic explains:
You can use NUMBER constants in arithmetic operations on date and timestamp values, but not interval values. Oracle internally converts timestamp values to date values and interprets NUMBER constants in arithmetic datetime and interval expressions as numbers of days. For example, SYSDATE + 1 is tomorrow. SYSDATE - 7 is one week ago. SYSDATE + (10/1440) is ten minutes from now.
(That's perhaps slightly misleading; SYSDATE + 1 is the same time tomorrow...)
It's common to see 1/24 used to represent an hour in calculations, 1/(24*60) to represent a minute, and 1/(24*60*60) to represent a second. Some people prefer that format - or 1/86400 - while some prefer interval '1' second or numtodsinterval(1, 'SECOND'), but they all mean the same in the end.
As Peter Lang said, 1/48 represents half an hour, as a fraction of one day. 1/24 is an hour, so 1/(2*24) is half an hour; it's the same as (1/2)*(1/24) if that helps.
You can compare two dates to see the fractional difference:
select 1/48, to_date('12:30', 'HH24:MI') - to_date('12:00', 'HH24:MI') as diff
from dual
1/48 DIFF
---------- ----------
.020833333 .020833333

Answer 2

The query I need to get the intervals within the pay periods of an overtime roster is done and I'll share it.
Here is my Frankenstein, which now HAS life. A curious thing, though ... I took a post literally and originally had my WHERE predicate as:
WHERE TRIM(UPPER(TO_CHAR(TO_DATE(TT.PERIOD_END, 'DD/MM/YYYY' ), 'DAY'))) = 'SATURDAY'
AND MOD(TT.PAY_PERIOD,2) <> 0
This curiously picked MONDAY PERIOD_END dates. I had to set the = to 'THURSDAY' to get Saturday dates. Well, taking out the un-needed TO_DATE() resolved this but I found it strange that it wigged out the way it did, offsetting what I was looking for by 2 days.
WITH TMP_TAB AS (
SELECT
START_TIME + (LEVEL - 1) AS PERIOD_START
,START_TIME + LEVEL + INTERVAL '13' DAY AS PERIOD_END
,LEVEL AS PAY_PERIOD
FROM (
SELECT
TO_DATE
(
TO_CHAR(
(
SELECT
CASE WHEN MOD(TO_NUMBER(TO_CHAR((SELECT MIN(ENTERED_DT) FROM OVTR_LOG) + DT1,'J')),2) = 0
THEN TRUNC((SELECT MIN(ENTERED_DT) FROM OVTR_LOG) + DT1 + 7)
ELSE TRUNC((SELECT MIN(ENTERED_DT) FROM OVTR_LOG) + DT1) END AS FIRST_PPE_DT
FROM (SELECT 7 - TO_NUMBER(TO_CHAR(MIN(ENTERED_DT),'D')) AS DT1 FROM OVTR_LOG)
),'DD/MM/YYYY'
), 'DD/MM/YYYY'
) START_TIME
,TO_DATE
(
TO_CHAR(
(
SELECT
CASE WHEN MOD(TO_NUMBER(TO_CHAR((SELECT MAX(ENTERED_DT) FROM OVTR_LOG) + DT1,'J')),2) = 0
THEN TRUNC((SELECT MAX(ENTERED_DT) FROM OVTR_LOG) + DT1 + 7)
ELSE TRUNC((SELECT MAX(ENTERED_DT) FROM OVTR_LOG) + DT1) END AS MOST_RECENT_PPE_DT
FROM (SELECT 7 - TO_NUMBER(TO_CHAR(MAX(ENTERED_DT),'D')) AS DT1 FROM OVTR_LOG)
),'DD/MM/YYYY'
), 'DD/MM/YYYY'
) END_TIME
FROM DUAL
)
CONNECT BY START_TIME + (LEVEL -1) < END_TIME
)
SELECT
TO_CHAR(TT.PERIOD_START, 'DD/MM/YYYY') PERIOD_START
,TO_CHAR(TT.PERIOD_END, 'DD/MM/YYYY') PERIOD_END
FROM TMP_TAB TT
--LEFT JOIN TO THE OTR
--ON WHATEVER
WHERE TRIM(UPPER(TO_CHAR(TT.PERIOD_END, 'DAY'))) = 'SATURDAY'
AND MOD(TT.PAY_PERIOD,2) <> 0
GROUP BY
TT.PERIOD_START
,TT.PERIOD_END
ORDER BY TT.PERIOD_START
This gives the following result set. My apologies for not being "in complete form" here -- as Alex suggested the help centre should be reviewed and it will be.
The original question included I had was what does / 48 do? It divides LEVEL according to the context LEVEL is used in, straight and simple.
Here is what I originally wanted as a result set and with the great help of Alex' query (thanks to Peter's original question) I got there. This should explain where I was trying to arrive.
PERIOD_START PERIOD_END
23/04/2011 07/05/2011
07/05/2011 21/05/2011
21/05/2011 04/06/2011
04/06/2011 18/06/2011
18/06/2011 02/07/2011
02/07/2011 16/07/2011
16/07/2011 30/07/2011
30/07/2011 13/08/2011
13/08/2011 27/08/2011
27/08/2011 10/09/2011
10/09/2011 24/09/2011
24/09/2011 08/10/2011
08/10/2011 22/10/2011
22/10/2011 05/11/2011
05/11/2011 19/11/2011
19/11/2011 03/12/2011
03/12/2011 17/12/2011
17/12/2011 31/12/2011
31/12/2011 14/01/2012
14/01/2012 28/01/2012
28/01/2012 11/02/2012
11/02/2012 25/02/2012
25/02/2012 10/03/2012
10/03/2012 24/03/2012
24/03/2012 07/04/2012
07/04/2012 21/04/2012
21/04/2012 05/05/2012
05/05/2012 19/05/2012
19/05/2012 02/06/2012
02/06/2012 16/06/2012