Stop recursive incestuous child parent relationship in mysql - php

Question

I am programming in PHP / MySQL / Javascript.
I have a list of parts which we want to link in a child / parent relationship with no limit on the amount of tiers.
When I am picking from a list of parts to add a child to a parent I limit the list of parts to exclude the parent itself, and any parts which are already children of that parent.
What I have discovered is that I also want to exclude the grandparents of the parent as otherwise we can get an incestuous relationship, which when I display the tree of parts will create an infinite loop.
Not only that but I cannot allow the child part to be a great grandparent of the parent or great great grandparent e.t.c.
Here is the SQL statement I use currently which I think could also be improved by using LEFT JOIN but I am not skillful enough with SQL at this point.
SELECT *
FROM sch_part_general
WHERE (sch_part_general.part_id <> $parentId)
AND (sch_part_general.part_id NOT IN
(SELECT part_id FROM sch_part_mapping WHERE parent_id = $parentId)
)
sch_part_general is a multi column table with all the parts, with part_id as the primary key.
sch_part_mapping is a two column mapping table with part_id (child) || parent_id (parent).
Could someone point me in the right direction with the SQL query? I am not keen on using a while loop to create the SQL statement as I think this will be quite inefficient but it is the only way I have considered might work so far.

Answer 1

MySQL doesn't have much (if any) support for hierarchical queries. If you want to stick to what is called theAdjacency List Model, all you can do is add a JOIN for each level you like to include. Needless to say this doesn't scale well.
On the other hand, if you can alter your Database Schema, I would suggest implementing the Nested Set Model.
A very good explantion of the Nested Set Model is presented in Mike Hillyer's blog
Limitations of the Adjacency List Model
Working with the adjacency list model in pure SQL can be difficult at
best. Before being able to see the full path of a category we have to
know the level at which it resides.
Nested Set Model
the concept of nested sets in SQL has been around for over a decade,
and there is a lot of additional information available in books and on
the Internet. In my opinion the most comprehensive source of
information on managing hierarchical information is a book called Joe
Celko’s Trees and Hierarchies in SQL for Smarties, written by a very
respected author in the field of advanced SQL, Joe Celko.

Answer 2

If you can't alter the schema, then there is no running away from looping as the answer from Lieven suggests.
if you can alter the schema, then maybe the following can also be enough for your case:
add a new column to the sch_part_mapping , lets call it "hierarchy_id". it is a value constructed to be unique int at the first time you start a totally new hierarchy (with the first grand grand grandest grandmost parent in any hierarchy - however its said in english) and is inserted to all lines belonging to a single hierarchy no matter at what level.
then, its easy to skip parents and grand parent found in the same hierarchy: to your sql above you can then add:
SELECT *
FROM sch_part_general
WHERE (sch_part_general.part_id <> $parentId)
AND (sch_part_general.part_id NOT IN
(SELECT part_id FROM sch_part_mapping WHERE parent_id = $parentId)
//addition here
and not exists (select * from sch_part_mapping where hierarchy_id= ? and parent_id = sch_part_general.part_id)
)
the question mark should be replaced with the relevant heirarchy id that you need to calculate.
EDIT: i missed that you have a variable for a specific parent ID, therefore the hierarchy_id can be calculated in the same query:
SELECT *
FROM sch_part_general
WHERE (sch_part_general.part_id <> $parentId)
AND (sch_part_general.part_id NOT IN
(SELECT part_id FROM sch_part_mapping WHERE parent_id = $parentId)
//addition here
and not exists (select * from sch_part_mapping where hierarchy_id= (select hierarchy_id from sch_part_mapping where parent_id = $parentId limit 1) and parent_id = sch_part_general.part_id)
)

Answer 3

With MySql/MariaDB you can use the Open Query Graph engine (http://openquery.com/graph/doc) which is a mysql plugin that lets you create a special table where you put the relationships, basically parentId and childId.
The magic is that you query this table with a special column latch depending of the value passed in the query will tell the OQGRAPH engine which command to execute. See the docs for details.
It handle not only tree (recursive 1-n relations), but graph data structures (recursive n-m relations) with weight (think for example that you want to store companies ownership, a company can have several subsidiaries and can also have several shareholders).