# 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.

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: