遞歸CTE生成額外記錄

Question

我有以下遞歸SQL查詢來生成日期中提供的任何月份的所有日期。

它總是會生成額外的日期（下個月的第一個日期）。

DECLARE @currentDate DATE = '2016-5-25'; 
WITH cte AS(
    SELECT DATEADD(DAY, -DATEPART(DAY, @currentDate) + 1, @currentDate) AS firstDay 
    UNION ALL 
    SELECT DATEADD(DAY, 1, firstDay) 
    FROM cte 
    WHERE DATEPART(MONTH, firstDay) = DATEPART(MONTH, @currentDate) 
) 
SELECT * FROM cte 

我在做什麼錯？

Answer 1

在Recursive CTE，遞歸查詢將首先執行，然後在Where子句驗證

這是正確的做法

DECLARE @currentDate DATE = '2016-5-25'; 

;WITH cte AS(
    SELECT dateadd(dd,1,eomonth(@currentDate,-1)) AS fd 
    UNION ALL 
    SELECT DATEADD(DAY, 1, fd) 
    FROM cte 
    WHERE fd < eomonth(@currentDate) 
) 
SELECT * FROM cte 

相反，可以使用符合表來做到這一點這是更好的方式性能相比Recursive CTE

;WITH e1(n) AS 
(
    SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL 
    SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL 
    SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 
), -- 10 
e2(n) AS (SELECT 1 FROM e1 CROSS JOIN e1 AS b), -- 10*10 
e3(n) AS (SELECT 1 FROM e1 CROSS JOIN e2), -- 10*100 
tally as (SELECT id = ROW_NUMBER() OVER (ORDER BY n) FROM e3) 
select 
dateadd(DD,ID-1,dateadd(dd,1,eomonth(@currentDate,-1))) as Dates 
from Tally 
where dateadd(DD,ID-1,dateadd(dd,1,eomonth(@currentDate,-1)))<=eomonth(@currentDate)