sql - 按计数更新列

Question

我在MySQL写了以下查询：

UPDATE mytable atable, 
(SELECT address_one, address_two, COUNT(*) cnt FROM table 
GROUP BY address_one, address_two) btable SET atable.address_count = btable.cnt 
WHERE atable.address_one = btable.address_one AND atable.address_two = btable.address_two

它计算表中列出的address_one和address_two的数量，并将数字存储在相应的address_count中。

但是，这在MySQL运行良好，但在SQL Server 。 如何为SQL Server修复此问题？

Answer 1

尝试这个：

update a
set a.address_count = b.cnt
from mytable a
join (
    select address_one,
        address_two,
        COUNT(*) cnt
    from mytable
    group by address_one,
        address_two
    ) b on a.address_one = b.address_one
    and a.address_two = b.address_two

附注 ：始终使用显式和现代连接语法，而不是旧的基于逗号的连接。

Answer 2

在SQL Server中，使用窗口函数更有效：

with toupdate as (
      select t.*,
             count(*) over (partition by address_one, address_two) as cnt
      from mytable t
     )
update toupdate
    set address_count = cnt;

在任一数据库中，您都可以使用相关子查询。 因此，以下代码应该适用于：

update mytable
    set address_count = (select count(*)
                         from mytable t2
                         where t2.address_one = mytable.address_one and
                               t2.address_two = mytable.address_two
                        );