SQL Server 2008 CTE Recursion

问题:

I am trying to perform what I believe is a difficult recursion using a CTE is SQL Server 2008. I can't seem to wrap my head around this one.

In the below examples you can assume a fixed depth of 3...nothing will ever be lower than that. In real life, the depth is "deeper" but still fixed. In the example I tried to simplify it some.

My input data is like the below.

ID     PARENT_ID       NAME          DEPTH
------------------------------------------
1      NULL            A             1
2      1               B             2
3      2               C             3
4      1               D             2

The output of my CTE should be the following table.

LEVEL1_ID    LEVEL2_ID    LEVEL3_ID    LEVEL1_NAME    LEVEL2_NAME    LEVEL3_NAME
--------------------------------------------------------------------------------
1            NULL         NULL         A              NULL           NULL
1            2            NULL         A              B              NULL
1            2            3            A              B              C
1            4            NULL         A              D              NULL

If I can get the ID columns in the output I can certainly map to names in a lookup table.

I am open to other ways of accomplishing this as well, including using SSIS.

回答1:

Not really all that hard to do:

;WITH cte AS
(
    SELECT CAST('/' + Name AS VARCHAR(50)) as 'CteName', ID
    FROM dbo.YourTable
    WHERE parent_id IS NULL

    UNION ALL

    SELECT CAST(cte.CteName + '/' + Name AS VARCHAR(50)), t.ID
    FROM dbo.YourTable t
    INNER JOIN cte ON t.parent_id = cte.id
)
SELECT cteName FROM cte
ORDER BY ID

Gives me an output of:

/A
/A/B
/A/B/C
/A/D

As a side-note: the "depth" could be easily computed by the CTE and you don't necessarily need to store that in your table (see the Level column I've added):

;WITH cte AS
(
    SELECT 
       CAST('/' + Name AS VARCHAR(50)) as 'CteName', ID, 
       1 AS 'Level'
    FROM dbo.YourTable
    WHERE parent_id IS NULL

    UNION ALL

    SELECT 
       CAST(cte.CteName + '/' + Name AS VARCHAR(50)), t.ID,
       cte.Level + 1 AS 'Level'
    FROM dbo.YourTable t
    INNER JOIN cte ON t.parent_id = cte.id
)
SELECT cteName FROM cte
ORDER BY Level, ID

回答2:

I don't remember of you can do a subquery in a cte.

I don't have a copy of sql server here but you can try with this code:

WITH cte(id, path, level)
AS
(
    SELECT id, '/' + name, level
    FROM yourtable
    WHERE level = 1

    UNION ALL

    SELECT y.id, c.name + '/' + y.name, y.level
    FROM yourtable y INNER JOIN
    cte c ON c.id = y.parent_id
    WHERE level = (SELECT max(level)+1 from cte)
)
SELECT path from cte

回答3:

;WITH Vals AS (
        SELECT  CASE DEPTH WHEN 1 THEN ID ELSE NULL END 'LEVEL1_ID ',
                CASE DEPTH WHEN 2 THEN ID ELSE NULL END 'LEVEL2_ID ',
                CASE DEPTH WHEN 3 THEN ID ELSE NULL END 'LEVEL3_ID ',
                CASE DEPTH WHEN 1 THEN NAME ELSE NULL END 'LEVEL1_NAME',
                CASE DEPTH WHEN 2 THEN NAME ELSE NULL END 'LEVEL2_NAME',
                CASE DEPTH WHEN 3 THEN NAME ELSE NULL END 'LEVEL3_NAME',
                ID 'PRMID'                
        FROM    #Table1
        WHERE   parentId IS NULL
        UNION ALL
         SELECT  CASE DEPTH WHEN 1 THEN ID ELSE LEVEL1_ID END 'LEVEL1_ID ',
                CASE DEPTH WHEN 2 THEN ID ELSE LEVEL2_ID END 'LEVEL2_ID ',
                CASE DEPTH WHEN 3 THEN ID ELSE LEVEL3_ID END 'LEVEL3_ID ',
                CASE DEPTH WHEN 1 THEN NAME ELSE LEVEL1_NAME END 'LEVEL1_NAME',
                CASE DEPTH WHEN 2 THEN NAME ELSE LEVEL2_NAME END 'LEVEL2_NAME',
                CASE DEPTH WHEN 3 THEN NAME ELSE LEVEL3_NAME END 'LEVEL3_NAME',
                ID 'PRMID'                               
        FROM    #Table1 inner join Vals on #Table1.parentId=PRMID

)

SELECT  * from Vals