SQL Server 逐行操作

Question

我正在尝试根据“值”列中的前一行记录在“增量”列中执行一些操作，

例如

row_num| Period | Measure | Decay
1      | Jan 08 | 10      | 
2      | Feb 08 | 18      | 
3      | Mar 08 | 7       | 
4      | Apr 08 | 67      | 

我想根据公式更新“衰减”列，

row_num| Period | Measure| Decay
1      | Jan 08 | 10     | = 10             -> first value in 'Measures'
2      | Feb 08 | 18     | = 10*0.5+18 = 23 -> previous decay record *0.5 + current measure
3      | Mar 08 | 7      | = 23*0.5+7  = 18.5
4      | Apr 08 | 67     | = 18.5*0.5+67 = 76.25

光标是否适用于此处？语法会是什么样子？ 谢谢

Answer 1

这是使用递归 CTE 的一个运行示例（另请注意，您的示例不正确）：

-- SO3192010

DECLARE @t AS TABLE (row_num int NOT NULL, Period varchar(6) NOT NULL, Measure float NOT NULL)
INSERT INTO @t VALUES (1, 'Jan 08', 10)
    ,(2, 'Feb 08', 18)
    ,(3, 'Mar 08', 7)
    ,(4, 'Apr 08', 67)

;WITH r AS (
    SELECT t.*, Measure AS Decay
    FROM @t AS t
    WHERE t.row_num = 1

    UNION ALL

    SELECT t.*, r.Decay * 0.5 + t.Measure AS Decay
    FROM r
    INNER JOIN @t AS t
        ON t.row_num = r.row_num + 1
)
SELECT *
FROM r
ORDER BY row_num​

Answer 2

UPDATE
   tbl_test 
SET
   col_name = 'xyq' 
FROM
   (
   SELECT
      *,
      ROW_NUMBER() OVER (ORDER BY name) AS rno
   FROM
     tbl_test  
   )tbl_test
WHERE
   rno = 3  

--rno is the rownumber that u want to update