Question

I have a table for my country territorial divisions, which does as follows :

• level 0: country
• level 1: district
• level 2: county
• level 3: parish

(the names were translated on Google)

This levels correspond to a parent => child pseudo relationship that goes as follows:

TABLE zones

id
parent_id
name

Sample data:

VALUES (1,0,'LEVEL 1')
VALUES (2,1,'LEVEL 2')
VALUES (3,2,'LEVEL 3')

So far so good. Now my problem is: how can I select all zones from a certain district (level 1) that are related to it?

Answer 1

This will get all information in joined format:

SELECT *
FROM zones l1
INNER JOIN zones l2 ON l2.parent_id = l1.id
INNER JOIN zones l3 ON l3.parent_id = l2.id
WHERE l1.id = <id>

However if you want to simply list all related zones (not including the district itself, but only county and parishes of that district), you’ll need to use a UNION:

SELECT l2.*
FROM zones l1
INNER JOIN zones l2 ON l2.parent_id = l1.id
WHERE l1.id = <id>

UNION ALL

SELECT l3.*
FROM zones l1
INNER JOIN zones l2 ON l2.parent_id = l1.id
INNER JOIN zones l3 ON l3.parent_id = l2.id
WHERE l1.id = <id>