T-SQL Window 带有 CTE 的函数，使用之前计算的值

Question

我尝试使用以下 CTE 查询计算结果字段中引用先前结果值的值：

WITH cteA (N, val1) AS (
    SELECT 1, 5.1 UNION
    SELECT 2, 6.5 UNION
    SELECT 3, 7.5 UNION
    SELECT 4, 4.6 UNION
    SELECT 5, 3.2
), cteB AS (
    SELECT *
        , val1 / LAG(val1) OVER (ORDER BY N) val2
        , (CASE N WHEN 1 THEN 100 END) result
    FROM cteA
)

SELECT *
FROM cteB

它 returns 结果字段上的 NULL 值意外：

https://i.stack.imgur.com/oeMcO.png

我需要帮助来获得预期值而不是 NULLs，如下所示：

https://i.stack.imgur.com/XprEW.png

Answer 1

你需要使用递归cte

with
cteR AS
(
    SELECT  N, val1, 
            val2   = convert(decimal(20,4), NULL), 
            result = convert(decimal(20,4), 100)
    FROM    cteA
    WHERE   N   = 1

    UNION ALL

    SELECT  a.N, a.val1, 
            val2   = convert(decimal(20,4), a.val1 / r.val1), 
            result = convert(decimal(20,4), a.val1 / r.val1 * r.result)
    FROM    cteR r
            INNER JOIN cteA a   on  r.N = a.N - 1

)
SELECT  *
FROM    cteR

T-SQL Window 带有 CTE 的函数，使用之前计算的值

T-SQL Window Functions with CTE, use previously computed value

tsql

sql-server

common-table-expression

window-functions