Question

我正在使用JOIN执行SQL UPDATE，但该JOIN可以匹配多个值。假设我们有以下表格：

    Table_1              Table_2
 col_a | col_b        col_a | col_b
---------------      ---------------
   1   |   A           1    |    X    
   2   |   B           1    |    Y   
   3   |   C           3    |    Z

我执行以下查询：

UPDATE
  t1
SET
  t1.col_b = t2.col_b
FROM
  Table_1 t1
  JOIN
  Table_2 t2 ON t1.col_a = t2.col_a;

结果如下：

    Table_1              Table_2
 col_a | col_b        col_a | col_b
---------------      ---------------
   1   |   X           1    |    X    
   2   |   B           1    |    Y   
   3   |   Z           3    |    Z

我需要做的是使用匹配的最后一个值更新Table_1;所以在这种情况下，我需要这个结果：

    Table_1              Table_2
 col_a | col_b        col_a | col_b
---------------      ---------------
   1   |   Y           1    |    X    
   2   |   B           1    |    Y   
   3   |   Z           3    |    Z

Answer 1

如果你有办法在Table_2中定义记录的顺序（最后是什么意思？），你可以使用窗口函数来过滤Table_2，只包括匹配的每组记录的最后一条记录：

UPDATE
  t1
SET
  t1.col_b = t2.col_b
FROM
  Table_1 t1
  JOIN
  (SELECT col_a, col_b,
          ROW_NUMBER() OVER (PARTITION BY col_a 
                             ORDER BY <order by field list goes here> DESC) AS RNo
   FROM Table_2) t2 ON t1.col_a = t2.col_a AND t2.RNo=1;

在特殊情况下，按字段顺序是col_b，那么您可以简单地使用（这适用于所有版本的SQL Server）：

UPDATE
  t1
SET
  t1.col_b = t2.col_b
FROM
  Table_1 t1
  JOIN
  (SELECT col_a, MAX(col_b) AS col_b
   FROM Table_2
   GROUP BY col_a) t2 ON t1.col_a = t2.col_a;

Answer 2

假设＆＃34;最后＆＃34; value是最大值，您可以通过cte汇总。

use test
go

/*
create table Table_1 (col_a int, col_b nchar(1));
create table Table_2 (col_a int, col_b nchar(1));

insert Table_1 values
(1,N'A'),
(2,N'B'),
(3,N'C');

insert Table_2 values
(1,N'X'),
(1,N'Y'),
(3,N'Z');
*/

with cte_maxT2 as
(
    select col_a, max(col_b) col_b
    from Table_2
    group by col_a
)
select t1.*,c.*
from Table_1 t1
join cte_maxT2 c on c.col_a=t1.col_a

使用匹配两个或多个值的JOIN进行SQL UPDATE

2 个答案: