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Answering questions asked on the site.

Michael asks:

I was wondering how to implement a hierarchical query in MySQL (using the ancestry chains version) for a single row, such that it picks up the parents (if any) and any children (if any).

The idea is, I want to be able to jump in at any point, provide an Id of some sort, and be able to draw out the entire hierarchy for that Id, both upwards and downwards.

We need to combine two queries here:

1. Original hierarchical query that returns all descendants of a given id (a descendancy chain)
2. A query that would return all ancestors of a given id (an ancestry chain)

An id can have only one parent, that's why we can employ a linked list technique to build an ancestry chain, like shown in this article:

• Sorting lists

Here's the query to to this (no functions required):

SELECT  @r AS _id,
         (
         SELECT  @r := parent
         FROM    t_hierarchy
         WHERE   id = _id
         ) AS parent,
         @l := @l + 1 AS lvl
 FROM    (
         SELECT  @r := 1218,
                 @l := 0,
                 @cl := 0
         ) vars,
         t_hierarchy h
WHERE    @r <> 0

To combine two queries, we can employ a simple UNION ALL.

The only problem that is left to preserve the correct level, since the ancestry chain query conts level backwards, and the hierarchical query will count it starting from zero.

Let's create a sample table and see what we get:
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