How to convert Dynamic 7 day rows into columns with t-sql - sql

Question

Background Info
I have a large table 400M+ rows that changes daily (one days data drops out an a new days data drops in) The table is partitioned on a 'day' field so there are 31 paritions.
Each row in the table has data similar to this:
ID, Postcode, DeliveryPoint, Quantity, Day, Month
1 SN1 1BG A1 6 29 1
2 SN1 1BG A1 1 28 1
3 SN1 1BG A2 2 27 1
4 SN1 1BG A1 3 28 1
5 SN2 1AQ B1 1 29 12
6 SN1 1BG A1 2 26 12
I need to pull out 7 days of data in the format:
Postcode, Deliverypoint, 7dayAverage, Day1,day2,Day3,Day4,Day5,Day6,Day7
SN1 1BG A1 2 0 1 2 1 3 4 0
I can easily extract the data for the 7 day period but need to create a columnar version as shown above.
I have something like this:
select postcode,deliverypoint,
sum (case day when 23 then quantity else 0 end) as day1,
sum (case day when 24 then quantity else 0 end) as day2,
sum(case day when 25 then quantity else 0 end) as day3,
sum(case day when 26 then quantity else 0 end) as day4,
sum(case day when 27 then quantity else 0 end) as day5,
sum(case day when 28 then quantity else 0 end) as day6,
sum(case day when 29 then quantity else 0 end) as day7,
sum(quantity)*1.0/#daysinweek as wkavg
into #allweekdp
from maintable dp with (nolock)
where day in (select day from #days)
group by postcode,deliverypoint
where #days has the day numbers in the 7 day period.
But as you can see, I've hard-coded the day numbers into the query, I want to get them out of my temporary table #days but can't see a way of doing it (an array would be perfect here)
Or a I going about this in completely the wrong way ?
Kind Regards
Steve

Answer 1

If I understand correctly, what I would do is:
Convert the day and month columns into datetime values,
Get the first day of the week and day of the weekday (1-7) for each date, and
Pivot the data and group by the first day of the week
see here: sqlfiddle

Answer 2

As utexaspunk suggested, Pivot might be the way to go. I've never been comfortable with pivot and have preferred to pivot it manually so I control how everything looks, so I'm using a similar solution to how you did your script to solve the issue. No idea how the performance between my way and utexaspunk's will compare.
Declare #Min_Day Integer = Select MIN(day) as Min_Day From #days;
With Day_Coding_CTE as (
Select Distinct day
, day - #Min_Day + 1 as Day_Label
From #days
)
, Non_Columnar_CTE as (
Select dp.postcode
, dp.deliverypoint
, d.day
, c.Day_Label
, SUM(quantity) as Quantity
From maintable dp with (nolock)
Left Outer Join #days d
on dp.day = d.day --It also seems like you'll need more criteria here, but you'll have to figure out what those should be
Left Outer Join Day_Coding_CTE c
on d.day = c.day
)
Select postcode
, deliverypoint
, SUM(Case
When Day_Label = 1
Then Quantity
Else 0
End) as Day1
, SUM(Case
When Day_Label = 2
Then Quantity
Else 0
End) as Day2
, SUM(Case
When Day_Label = 3
Then Quantity
Else 0
End) as Day3
, SUM(Case
When Day_Label = 4
Then Quantity
Else 0
End) as Day4
, SUM(Case
When Day_Label = 5
Then Quantity
Else 0
End) as Day5
, SUM(Case
When Day_Label = 6
Then Quantity
Else 0
End) as Day6
, SUM(Case
When Day_Label = 7
Then Quantity
Else 0
End) as Day7
, SUM(Quantity)/#daysinweek as wkavg
From Non_Columnar_CTE
Group by postcode
deliverypoint