3

My table design is

users (id, username..., parent_id)

Data is

id    username      parent_id
-------------------------------
1  |  admin       | null
2  |  reseller 1  | 1
3  |  client 1    | 1
4  |  reseller 2  | 1
5  |  reseller 3  | 2
6  |  reseller 4  | 2
7  |  client 2    | 5
8  |  client 3    | 6

I want to get all descendants of id 1

I studied Adjacency List, Nested List & Closure table design, but came to conclusion that Session based Adjacency List can be better in my situation.

I found an example at https://stackoverflow.com/questions/8104187/hierarchical-queries-in-mysql

SELECT  @id :=
        (
        SELECT  senderid
        FROM    mytable
        WHERE   receiverid = @id
        ) AS person
FROM    (
        SELECT  @id := 5
        ) vars
STRAIGHT_JOIN
        mytable
WHERE   @id IS NOT NULL

I tried to modify this query to get my result like this

SELECT  group_concat(@id :=
        (
        SELECT  id
        FROM    users
        WHERE   parent_id = @id
        )) AS u
FROM    (
        SELECT  @id := 1
        ) vars
STRAIGHT_JOIN
        users
WHERE   @id IS NOT NULL

But it is not working. Here is the SQLFiddle

Need help in making the query work.

  • The STRAIGHT_JOIN is a "cross join" in your case. Why do you think you need that? – Rick James Jan 15 '16 at 19:27
  • I don't know exactly what the query does. Just copied it from another solution, modified it to adapt to my scenario. It works, if a parent has only one child, but fails in one to many relation, as the sub query in that case has more than one row – Ehs4n Jan 15 '16 at 19:34
  • I would (and have done) write application code to traverse the tree as needed. It's only a few SELECTs to go up to the root or down to the descendants. – Rick James Jan 15 '16 at 20:01
4

The query in the linked answer is a "hack" as the OP there clearly describes in a comment and can stop working any time with a MySQL upgrade as the exact behaviour of session variables is not documented.

Aside from that, the question there was about finding all ancestors, not all descendants. In adjacency lists, a node has maximum one parent but possibly many children. So this query/hack will not work in your case.

The Adjacency List model is the simplest of all hierarchy models in SQL. The other 3 (Nested Sets, Evaluated Path, Closure Table) are more complicated because - in a way - store redundant information. In exchange, some queries are far more easier to write. In any of the other 3 models, the solution would be really simple and wouldn't need a recursive query.

So, it seems you have two options:

  • either evaluate your decision to use the Adjacency List model and use a different one (or a combination of models)
  • write a procedure / function that solves the issue. @RolandoDBA has an old answer on this site, that provides several procedures that solve this (and related) problems. I'm not sure how efficient they are but they surely work:
    Find highest level of a hierarchical field: with vs without CTEs.

You'll find more useful information in the links:

  • Creating custom function as mentioned by RolandoMySQLDBA worked. I have mentioned below the answer. – Ehs4n Jan 17 '16 at 7:11
0

Finally, I created a function as suggested by @RolandoDBA at Find highest level of a hierarchical field: with vs without CTEs

DELIMITER $$

DROP FUNCTION IF EXISTS `siblings` $$
CREATE FUNCTION `siblings` (GivenID INT) RETURNS varchar(1024) CHARSET latin1
DETERMINISTIC
BEGIN

    DECLARE rv,q,queue,queue_children VARCHAR(1024);
    DECLARE queue_length,front_id,pos INT;

    SET rv = '';
    SET queue = GivenID;
    SET queue_length = 1;

    WHILE queue_length > 0 DO
        SET front_id = FORMAT(queue,0);
        IF queue_length = 1 THEN
            SET queue = '';
        ELSE
            SET pos = LOCATE(',',queue) + 1;
            SET q = SUBSTR(queue,pos);
            SET queue = q;
        END IF;
        SET queue_length = queue_length - 1;

        SELECT IFNULL(qc,'') INTO queue_children
        FROM (SELECT GROUP_CONCAT(id) qc
        FROM `users` WHERE parent_id = front_id) A;

        IF LENGTH(queue_children) = 0 THEN
            IF LENGTH(queue) = 0 THEN
                SET queue_length = 0;
            END IF;
        ELSE
            IF LENGTH(rv) = 0 THEN
                SET rv = queue_children;
            ELSE
                SET rv = CONCAT(rv,',',queue_children);
            END IF;
            IF LENGTH(queue) = 0 THEN
                SET queue = queue_children;
            ELSE
                SET queue = CONCAT(queue,',',queue_children);
            END IF;
            SET queue_length = LENGTH(queue) - LENGTH(REPLACE(queue,',','')) + 1;
        END IF;
    END WHILE;

    RETURN rv;

END $$

And then for getting siblings / children of a parent, a simple call using the above created function, siblings is enough.

SELECT siblings(id) AS `siblings` from `users` where `id` = 1

This returns result as

siblings
---------------
2,3,4,5,6,7,8

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