Historic and avg data in sql - sql

Question

I want to get report data in below scenarios using the sample table provided below(data is huge in my db)
List item(same week, prior year) sales and (same day, same week, prior year) sales.
Rolling 6 month avg weekly selling
id
date_week
date_value
sales
Item1
2020/01-04
20200120
230
Item2
2020/06-03
20200608
23.0
Item3
2019/11-03
20191111
null
Item4
2020/07-04
20200720
123
Item5
2019/08-01
20190729
456
Item6
2019/09-03
20190909
1234
Item7
2020/06-02
20200601
4556
Item8
2020/09-01
20200824
23
Item9
2021/09-02
20210906
1223
in above table date_week is year/month_week ( so here i get the week number)
Am trying the below query to achieve
SELECT
DATEPART(week, date_value) AS Week,
id ,
sum(sales) AS sales
FROM table
WHERE date_value <= date_value
AND date_value < DATEADD(year, 1, date_value)
GROUP BY DATEPART(week, date_value), id
ORDER BY DATEPART(week, date_value);
Please suggest me how to achieve the scenarios am looking for.

Answer 1

You can do these with a join. First I would separate my date columns to Year | Month | Week | Day. If this is the only format you have available for your dates, you can use left(), right() functions. Always better to have datetime format tho.
After your query can look like:
SELECT t1.Year, t1.Week, t1.Id, t1.Sales, t2.Sales as Last_year_this_week_sales
from (
SELECT
cast(right(date_value,2) as int) AS Week,
cast(left(date_value,4) as int) as Year
id ,
sum(sales) AS sales
FROM table
GROUP BY right(date_value,2),
left(date_value,4), id ) t1
left join (
SELECT
cast(right(date_value,2) as int) AS Week,
cast(left(date_value,4) as int) as Year
id ,
sum(sales) AS sales
FROM table
GROUP BY right(date_value,2),
left(date_value,4), id ) t2 ON t1.week = t2.week and t2.year = t1.year-1 and t1.id = t2.id';
I am assuming you want to have results on id level. if not, you need to remove it from your grouping and join. And you can do the same for the daily results replacing week with day.
You can do with a subquery - if you need to take the average of weekly totals in the last 6 months. And again, if you need it on item level, keep the id in your select and group by statements.
If not, you can do:
SELECT avg(sales) as Sales_avg
FROM
(SELECT
cast(right(date_value,2) as int) AS Week,
cast(left(date_value,4) as int) as Year,
sum(sales) AS sales
FROM table
where date_value>='20210101' --replace with the date you need
GROUP BY right(date_value,2),
left(date_value,4) ) t1;