SQL Server: advanced update query, Sql Server

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declare @stock TABLE([StockDate] [datetime] NULL, [stock] [int]);
    insert into @stock values 
     ('20170320',12)
    ,('20170321',-3)
    ,('20170322',-1)
    ,('20170323',-5)
    ,('20170324',0)
    ,('20170325',-4)
    ,('20170326',-7)
    ,('20170327',0); 
    declare @orderpoint table ([StockDate] [datetime] NULL, [stock] [int]);
    insert into @orderpoint values 
     ('20170323',10)
    ,('20170326',10);

select 
        stockdate = convert(varchar(10),stockdate,120)
      , stock
      , RunningTotal = sum(stock) over (order by stockdate, stock desc rows unbounded preceding)
    from (
        select s.stockdate, stock = s.stock+isnull(o.stock,'')
        from @stock s
          left join @orderpoint o
            on s.stockdate = o.stockdate
       ) t
    order by stockdate;
    
    ;with cte as (
    select 
        stockdate = convert(varchar(10),stockdate,120)
      , stock
      , RunningTotal = sum(stock) over (order by stockdate, stock desc rows unbounded preceding)
    from (
        select s.stockdate, stock = s.stock+isnull(o.stock,'')
        from @stock s
          left join @orderpoint o
            on s.stockdate = o.stockdate
       ) t
    )
    update s
      set s.stock = cte.RunningTotal
    from @stock s
      inner join cte
        on s.stockdate = cte.stockdate;
    
    select 
        stockdate = convert(varchar(10),stockdate,120)
      , stock
    from @stock;

View schema
Execution time: 0,03 sec, rows selected: 16, rows affected: 18, absolute service time: 0,22 sec