从评论看来，你想更新我添加了更新

with cte as
    (
    select customer, product,
           (case when row_number() over (partition by product order by customer) = 1 then monthlytargetqty end) as monthlytargetqty
    from t
    )
  update a 
  set a.MontylyTargetQty= b.monthlytargetqty
  from ProductAnalysisTable a join cte on
a.customer=cte.customer and a.product=b.product

顺便说一句，第一部分是戈登先生，所以接受他的回答

你似乎想要：

select customer, product,
       (case when row_number() over (partition by product order by customer) = 1 then monthlytargetqty end) as monthlytargetqty
from t
order by product, customer;

它使用 row_number() 为每个客户定义第一行，然后定义 case 表达式来保留该行上所需的值。请注意 order by 符合 partition by / order by 为了 row_number() .
编辑：
如果要更新现有表（我觉得这是个非常糟糕的主意），可以执行以下操作：

update t join
       (select product, min(customer) as min_customer
        from t
        group by product
       ) tt
       on t.product = tt.product and t.customer <> tt.min_customer
    set monthlytargetqty = 0;