Identify the period start date for each day in a range of dates

Question

When selecting a range of dates, I need to identify the specific period start date for each sales date. The periods are every two weeks and all I know is the start date for the most recent "previous period". I also know that the two week periods are calculated every two weeks from 10th September 2012.

For example, give the date range between '2013-07-03' and '2013-09-08':

emp|salesdate  |sales |prevperiod |item
123|2013-07-27 |12000 |2013-08-12 |A12
123|2013-07-27 |1800  |2013-08-12 |A17
123|2013-07-28 |400   |2013-08-12 |B03
227|2013-07-27 |1500  |2013-08-12 |A17

How would my SQL work to include the sales date's period start date:

emp|salesdate  |sales |prevperiod |item |salesdateperiodstart
123|2013-07-27 |12000 |2013-08-12 |A12  |2013-07-15
123|2013-07-27 |1800  |2013-08-12 |A17  |2013-07-15
123|2013-07-28 |400   |2013-08-12 |B03  |2013-07-15
123|2013-07-29 |400   |2013-08-12 |B03  |2013-07-29
227|2013-07-27 |1500  |2013-08-12 |A17  |2013-07-15
123|2013-07-28 |400   |2013-08-12 |B03  |2013-07-15

Basically a period runs for two weeks and has been running since 10th September 2012. Given a date range for sales, I need to identify the closes period start date, for each sales day.

Answer 1

Basically what you have to do is to

• Solution 1: Calculate whole weeks from '2012-09-10' and then add it to '2012-09-10'
• Solution 2: Calculate remainder when calculating whole weeks from '2012-09-10' and then substract it from salesdate:

-

select
    emp, salesdate, sales, prevperiod, item,
    dateadd(day, -datediff(day, '20120910', salesdate) % 14, salesdate) as salesdateperiodstart1,
    dateadd(day, (datediff(day, '20120910', salesdate) / 14) * 14, '20120910') as salesdateperiodstart2
from Table1

=> sql fiddle demo

Answer 2

Your start date is Monday, September 10, 2012, and your periods run for fourteen days. So, the next period starts Monday, September 24, 2012, and so forth.

The following little formula will take any date (or datetime) and convert it -- we could say truncate it -- to the first date of the reporting period in which it lies.

 SELECT FROM_DAYS(TO_DAYS(thedate) -MOD(TO_DAYS(thedate) -9, 14))

This is a little obscure and nasty, but it is based on How to group by week in MySQL? , and is extensively tested for such things as ends-of-year rollovers.

Here's a little query in a sql-fiddle that will summarize sales by reporting period start date. http://sqlfiddle.com/#!2/5193c/1/0

I'm not sure what you need prevperiod for. But you don't need it for this computation.