SQL CTE counting childs recursion

问题

I'd like (using cte) to count children in table in that way to have at parent level number of all children including theirs children. Is there any sample available?

回答1:

CREATE TABLE t_parent (id INT NOT NULL PRIMARY KEY, parentID INT NOT NULL)

INSERT
INTO    t_parent
VALUES  (1, 0)
INSERT
INTO    t_parent
VALUES  (2, 1)
INSERT
INTO    t_parent
VALUES  (3, 1)
INSERT
INTO    t_parent
VALUES  (4, 2)
INSERT
INTO    t_parent
VALUES  (5, 1)
INSERT
INTO    t_parent
VALUES  (6, 5)
INSERT
INTO    t_parent
VALUES  (7, 5);

WITH    q AS
        (
        SELECT  id, parentId
        FROM    t_parent
        UNION ALL
        SELECT  p.id, p.parentID
        FROM    q
        JOIN    t_parent p
        ON      p.id = q.parentID
        )
SELECT  id, COUNT(*)
FROM    q
GROUP BY
        id

来源：https://stackoverflow.com/questions/2191847/sql-cte-counting-childs-recursion