SQL SERVER recursive query (1) - common methods (CTE writing, functions)

      In actual query, we often encounter examples that require recursive query. The version before SQL SERVER 2005 can be implemented by function method, and after SQL SERVER 2005, CTE can be used (Common Table Expression is introduced after SQL SERVER 2005 version). a feature of ) to query.

--Test Data
if not object_id(N'T') is null
	drop table T
Go
Create table T([id] int,[pid] int,[num] int)
Insert T
select 1,0,1 union all
select 2,1,1 union all
select 3,2,1 union all
select 4,2,1 union all
select 5,2,1 union all
select 6,3,1 union all
select 7,3,1
Go
-- end of test data

      We want to accumulate results upwards, calculate how many subsets are at each level, function way, create a new function:

IF OBJECT_ID('dbo.f_GetChildren') IS NOT NULL
    DROP FUNCTION dbo.f_GetChildren
GO
CREATE FUNCTION f_GetChildren
    (
      @id INT ,
      @pid INT ,
      @num INT
    )
RETURNS @tab TABLE
    (
      [id] INT ,
      [pid] INT ,
      [num] INT
    )
AS
    BEGIN
        INSERT  @tab
                SELECT  @id ,
                        @pid ,
                        @on one
        WHILE @@rowcount > 0
            BEGIN               
                INSERT  @tab
                        SELECT  T.id ,
                                T.pid ,
                                T.num
                        FROM    T
                                JOIN @tab t1 ON T.pid = t1.id
                        WHERE   NOT EXISTS ( SELECT *
                                             FROM   @tab
                                             WHERE  T.id = [@tab].id )
            END
        RETURN  
    END
	GO

      transfer:

SELECT  id ,
        ( SELECT    SUM(num)
          FROM      f_GetChildren(id, pid, num)
        ) AS sumnum
FROM    T

      result:

      Ways to use CTE:

;WITH cte AS (
SELECT *,id AS sumid FROM dbo.T
UNION ALL
SELECT T.*,cte.sumid FROM T JOIN cte ON T.pid=cte.ID
)
SELECT sumid,SUM(num) AS sumnum FROM cte GROUP BY sumid

      result:

      The above are two basic implementations of recursion, function and CTE form.