T-SQL Select minimum values from different records of the table - sql

Question

Let's say I have a query in which I count the number of events per day:
**Date** **NumberOfEvents**
2017-11-1 7
2017-11-2 11
2017-11-3 3
...
2017-11-8 24
2017-11-9 6
2017-11-10 10
2017-11-11 9
...
2017-11-22 22
2017-11-23 11
2017-11-24 14
2017-11-25 17
...
2017-11-28 16
2017-11-29 21
2017-11-30 6
...
Then let's say I would define a variable #StartingDay ='2017-11-3'
I would like to get a query with the minimum value for the same week day +-1 day for the next 4 weeks after #StartingDay, p.ex:
**Period** **DateWithMin** **MinNumberOfEvents**
2017-11-09 To 2017-11-11 2017-11-9 6
2017-11-16 To 2017-11-18 2017-11-17 8
2017-11-23 To 2017-11-25 2017-11-23 11
2017-11-30 To 2017-12-02 2017-11-30 6
I believe I would have to cycle through the different periods to search for the min, but I can't find a way to cycle.

Answer 1

Another way to do this is to generate the From and To dates with a recursive CTE, apply a Row_Number() to the results to find the Min per grouping, and select only those results:
Declare #StartingDay Date = '2017-11-03',
#NumWeeks Int = 4
;With Dates As
(
Select DateFrom = DateAdd(Day, -1, DateAdd(Week, 1, #StartingDay)),
DateTo = DateAdd(Day, 1, DateAdd(Week, 1, #StartingDay))
Union All
Select DateFrom = DateAdd(Week, 1, DateFrom),
DateTo = DateAdd(Week, 1, DateTo)
From Dates
Where DateTo < DateAdd(Day, 1, DateAdd(Week, #NumWeeks, #StartingDay))
), Results As
(
Select PeriodFrom = D.DateFrom,
PeriodTo = D.DateTo,
NumberOfEvents = Y.NumberOfEvents,
RN = Row_Number() Over (Partition By D.DateFrom, D.DateTo
Order By Y.NumberOfEvents),
Date = Y.Date
From YourTable Y
Join Dates D On Y.Date Between D.DateFrom And D.DateTo
)
Select PeriodFrom,
PeriodTo,
DateWithMin = Date,
MinNumberOfEvents = NumberOfEvents
From Results
Where RN = 1

Answer 2

You can use modulo and date arithmetic to get the time periods and groups:
select min(date), max(date), min(NumberOfEvents)
from t
where (datediff(day, #startingday, date) % 7) in (0, 1, 6) and
date > dateadd(day, 1, #startingday) and
date <= dateadd(day, 4 * 7 + 1, #startingday)
group by (datediff(day, #startingday, date) + 1) / 7;
Getting the date of the minimum event is more troublesome. Here is one method:
select min(date), max(date), min(NumberOfEvents),
max(case when seqnum = 1 then date end) as date_at_min
from (select t.*, v.grp,
row_number() over (partition by grp order by numberofevents) as seqnum
from t cross apply
(values ((datediff(day, #startingday, date) + 1) / 7)) v(grp)
) t
where (datediff(day, #startingday, date) % 7) in (0, 1, 6) and
date > dateadd(day, 1, #startingday) and
date <= dateadd(day, 4 * 7 + 1, #startingday)
group by grp;