How can I generate 4-week ranges of dates? - sql

Question

I have to generate date range for 1 year in gap of 4 weeks ( or 28 days ) from a fixed date backward and going forward. For example I have DATE '2016-02-20'. I need to generate the below.
Start date = Sunday , End-date = Saturday
No Start_date End_date
==== ========= =======
1 1/24/2016 2/20/2016
2 12/27/2015 1/23/2016
3 11/29/2015 12/26/2015
4 .....
13 2/22/2015 3/21/2015
14 1/25/2015 2/21/2015
But,when 03/20/2016(Sunday) comes,it should add
1. 2/21/2016 3/19/2016 & remove 14. 1/25/2015 2/21/2015
and so on after every 4 weeks.
I have written the below, but I need help to iterate in minimal code( if possible.)
SELECT LEVEL,
DATE '2016-02-20'-27*LEVEL-LEVEL+1 AS start_date,
DATE '2016-02-20'-28*(LEVEL-1) AS end_date
FROM DUAL
Connect BY LEVEL < 15;

Answer 1

It seems you want to have a rolling window of a year's worth of four-week ranges, based from the current date. To do that you need a fixed known period start (or end) date you can work from. Picking one that happens to be January 1st you can do:
SELECT DATE '2012-01-01' + (28 * (LEVEL - 1)) AS start_date,
DATE '2012-01-01' + (28 * LEVEL) - 1 AS end_date
FROM DUAL
CONNECT BY DATE '2012-01-01' + (28 * LEVEL) - 1 <= TRUNC(sysdate)
Which will find 54 periods up to today. On March 21st it will find 55 periods, etc. You only want those that are in the last year, so use that as an inline view and restrict the range:
SELECT ROW_NUMBER() OVER (ORDER BY start_date DESC) AS no, start_date, end_date
FROM (
SELECT DATE '2012-01-01' + (28 * (LEVEL - 1)) AS start_date,
DATE '2012-01-01' + (28 * LEVEL) - 1 AS end_date
FROM DUAL
CONNECT BY DATE '2012-01-01' + (28 * LEVEL) - 1 <= TRUNC(sysdate)
)
WHERE end_date >= ADD_MONTHS(TRUNC(sysdate), -12)
ORDER BY start_date DESC;
NO START_DATE END_DATE
---------- ---------- ----------
1 01/24/2016 02/20/2016
2 12/27/2015 01/23/2016
3 11/29/2015 12/26/2015
...
11 04/19/2015 05/16/2015
12 03/22/2015 04/18/2015
13 02/22/2015 03/21/2015
The ROW_NUMBER() just generates your NO column, as the LEVEL is now in the wrong order.
If you always want exactly 14 rows in the result set you can move the ROW_NUMBER() into the inline view:
SELECT no, start_date, end_date
FROM (
SELECT ROW_NUMBER() OVER (ORDER BY LEVEL DESC) AS no,
DATE '2012-01-01' + (28 * (LEVEL - 1)) AS start_date,
DATE '2012-01-01' + (28 * LEVEL) - 1 AS end_date
FROM DUAL
CONNECT BY DATE '2012-01-01' + (28 * LEVEL) - 1 <= TRUNC(sysdate)
)
WHERE no <= 14
ORDER BY no;
NO START_DATE END_DATE
---------- ---------- ----------
1 01/24/2016 02/20/2016
2 12/27/2015 01/23/2016
3 11/29/2015 12/26/2015
...
12 03/22/2015 04/18/2015
13 02/22/2015 03/21/2015
14 01/25/2015 02/21/2015
14 rows selected