Mysql – Multi level hierarchy relationship

database-designhierarchyMySQL

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.

Best Answer

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:

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