T-SQL go through Dates

Question

I have the following table:

Column1     Column2   Column3
04/07/2019    1
04/08/2019    2
04/09/2019    8
04/10/2019    9
04/11/2019    15
04/12/2019    16
04/13/2019    5
04/14/2019    6 
04/15/2019    8
04/16/2019    9 
04/17/2019    10 
04/18/2019    11  
04/19/2019    5 
04/20/2019    5 
04/21/2019    8    
04/22/2019    8 
04/23/2019    9 
04/24/2019    10 
04/25/2019    11  
04/26/2019    12 
04/27/2019    10 

I need to find out a way to iterate through the values in column one and identify weeks which should start from Saturday - Sunday. So, in this example one iteration should be from the 14 - 20th. Or another iteration would be from the 7th through the 13th which is Saturday - Sunday. Then After identifying each week, I need to do some calculation on the other columns.The calculation would be updating Column3 if the total amount for Column2 within 1 week (Based on Column1 Saturday to Sunday) exceeds 40 or not. Then the same for the next iteration of week (Saturday - Sunday).

Desired Results:

Column1    Column2   Column3
04/07/2019    1        56
04/08/2019    2        56
04/09/2019    8        56
04/10/2019    9        56
04/11/2019    15       56
04/12/2019    16       56
04/13/2019    5        56
04/14/2019    6        54
04/15/2019    8        54
04/16/2019    9        54
04/17/2019    10       54
04/18/2019    11       54
04/19/2019    5        54
04/20/2019    5        54
04/21/2019    8        68
04/22/2019    8        68
04/23/2019    9        68
04/24/2019    10       68
04/25/2019    11       68
04/26/2019    12       68
04/27/2019    10       68

Please note: The data can range from 3 weeks to a few month. So, the code needs to capture the weeks for any specific range.

Answer 1

You can use datepart() to get the week of a date. You can then use the week to partition by in a windowed sum() . From there you can UPDATE the table joining a derived table that gets the sum like mentioned before. To make sure the week begins on Sunday issue a SET DATEFIRST 7 before the UPDATE .

SET DATEFIRST 7;
UPDATE t1
       SET t1.column3 = t3.column3
       FROM elbat t1
            INNER JOIN (SELECT t2.column1,
                               sum(t2.column2) OVER (PARTITION BY datepart(week, t2.column1)) column3
                               FROM elbat t2) t3
                       ON t3.column1 = t1.column1;

db<>fiddle