Stop recursive incestuous child parent relationship in mysql
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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.