Question

我有一个函数用来生成记录id，我想用CTE来获取批量记录id。现在递归CTE如下

  with T as (
    select 
        dbo.Ufn_GetRecordId() AS recordId
    union all
    SELECT
        dbo.Ufn_GetRecordId() AS recordId
    FROM T

)select * from T
OPTION (MaxRecursion 0);

但是，此查询不会终止。如何限制CTE的数量？（例如，如果我在T中只需要3行）

Answer 1

您可以尝试以下内容。取自SQL Server: How to limit CTE recursion to rows just recursivly added?

的想法

with T as (
    select 
        dbo.Ufn_GetRecordId() AS recordId, 1 as testnum
    union all
    SELECT
        dbo.Ufn_GetRecordId() AS recordId, testnum + 1
    FROM T
    WHERE testnum < 3

)select * from T
OPTION (MaxRecursion 0);

这将限制为3个返回的行。

Answer 2

这是一种使用递归CTE生成N行的相当标准的方法。

WITH T 
     AS (SELECT 1 AS Dummy 
         UNION ALL 
         SELECT Dummy + 1 
         FROM   T 
         WHERE  Dummy < 3) 
SELECT dbo.Ufn_GetRecordId() AS RecordId 
FROM   T;

如果您需要生成超过100个数字，那么您需要OPTION (MAXRECURSION 0)（或某个合适的值而不是0）。

如何限制递归CTE行数

2 个答案: