如何在 SQL 中存储递归 CTE 的结果？答案

【问题标题】：How can I store the result of a recursive CTE in SQL?如何在 SQL 中存储递归 CTE 的结果？
【发布时间】：2018-04-12 10:41:31
【问题描述】：

如果我有这样的递归 CTE，如何将结果存储在变量中？

WITH MyCTE (ChildID, ParentID)
AS
(
   SELECT ID, ParentID From MyTable
    WHERE ID = 1
   UNION ALL
    SELECT MT.ID, MT.ParentID 
    FROM MyCTE Child
    JOIN MyTable MT ON MT.ID = Child.ParentID
    WHERE Child.ParentID IS NOT NULL
)
SELECT COUNT(*)
FROM MyCTE
OPTION (MAXRECURSION 20)

我尝试了以下方法，但 SSMS 抱怨这两个选项的语法：

DECLARE @numberOfRecords int = (
WITH MyCTE (ChildID, ParentID)
AS
(
   SELECT ID, ParentID From MyTable
    WHERE ID = 1
   UNION ALL
    SELECT MT.ID, MT.ParentID 
    FROM MyCTE Child
    JOIN MyTable MT ON MT.ID = Child.ParentID
    WHERE Child.ParentID IS NOT NULL
)
SELECT COUNT(*)
FROM MyCTE
OPTION (MAXRECURSION 20))

和

WITH MyCTE (ChildID, ParentID)
AS
(
   SELECT ID, ParentID From MyTable
    WHERE ID = 1
   UNION ALL
    SELECT MT.ID, MT.ParentID 
    FROM MyCTE Child
    JOIN MyTable MT ON MT.ID = Child.ParentID
    WHERE Child.ParentID IS NOT NULL
)
DECLARE @numberOfRecords int = (
SELECT COUNT(*)
FROM MyCTE
OPTION (MAXRECURSION 20))

【问题讨论】：

在 CTE 之前先声明你的变量并写 select @Variable = count(*) from myCTE option(maxrecursion 20)

标签： sql sql-server common-table-expression recursive-query

【解决方案1】：

你可以先声明变量，然后赋值：

DECLARE @numberOfRecords int;

WITH MyCTE (ChildID, ParentID) AS (
      SELECT ID, ParentID
      FROM MyTable
      WHERE ID = 1
      UNION ALL
      SELECT MT.ID, MT.ParentID 
      FROM MyCTE Child JOIN
           MyTable MT
           ON MT.ID = Child.ParentID
      WHERE Child.ParentID IS NOT NULL
     )
SELECT @numberOfRecords = COUNT(*)
FROM MyCTE
OPTION (MAXRECURSION 20);

【讨论】：