title: TSQL - Query to get sum of last 3 months QTY
body: <p>I am working on a query to get sum of (current + last 2 month) QTY based on a date field in the table.</p>
  <pre><code>CREATE table #temp_Sales
  ( Client varchar(10),
  Sale_Month Date,
  Qty int)
  
  Insert into #temp_Sales VALUES
    ( 'AAAA', '2022-06-01',  5 ),
    ( 'AAAA', '2022-05-01',  10 ),
    ( 'AAAA', '2022-05-01',  2 ),
    ( 'AAAA', '2022-04-01',  5 ),
    ( 'AAAA', '2022-02-01',  15),
    ( 'BBBB', '2022-05-01',  2 ),
    ( 'BBBB', '2022-04-01',  4),
    ( 'BBBB', '2022-03-01', 6 ),
    ( 'BBBB', '2022-03-01', 10 ),
    ( 'BBBB', '2022-01-01', 6 ),
    ( 'BBBB', '2021-10-01', 10),
    ( 'BBBB', '2021-09-01', 2 ),
    ( 'BBBB', '2021-11-01', 4 ),
    ( 'BBBB', '2021-08-01', 6),
    ( 'BBBB', '2021-07-01',  8 ),
    ( 'CCCC', '2021-11-01', 2 ),
    ( 'CCCC', '2021-10-01', 3 ),
    ( 'CCCC', '2021-09-01', 30 ),
    ( 'CCCC', '2021-06-01', 4 )
  
  </code></pre>
  <p>Sample data:</p>
  <p><a href="https://i.stack.imgur.com/jqQqP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jqQqP.png" alt="enter image description here" /></a></p>
  <p>Expected Output:</p>
  <p><a href="https://i.stack.imgur.com/72lp5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/72lp5.png" alt="enter image description here" /></a></p>
  <p>The Sale_month is not consecutive and same month can appear more than once for a client in the table.</p>
  <p>Example : For the Client AAAA and Sale Month 2022-06-01 the qty should include the sum(QTY) of current and last 2 months ( 2022-06-01,2022-05-01 and 2022-04-01) for that client. QTY = 5 + 10 + 2 + 5 = 22</p>
  <p>For the client BBBB and Sale month 2022-03-01 . QTY =  6 + 10 + 6 = 22</p>
  <pre><code>  ;With da AS
    (SELECT *, DATEADD(MM,-2,Sale_month)as last_two_Months FROM #temp_Sales)
  
    Select Client,Sale_month,Sum(qty) from da
    WHERE Sale_month Between last_two_Months and Sale_month
    GROUP BY Client,Sale_month
    Order by client
  
  </code></pre>
  <p>Tried the above query. But not working as expected not sure how to group by using last_two_Months and Sale_month. Any help is much appreciated.</p>