Hi,
I'm playing around with putting a hierarchy of items into the database. But for
some reason I'm having a mental block understanding the following:
I have a table category with id and parent_id implementing the typical
adjacency model.
To get the first two levels of the hierarchy I use:
SELECT t1.name as lev1, t2.name as lev2
FROM category t1
LEFT JOIN category t2 ON t2.parent = t1.id
WHERE t1.name = 'ROOT'
;
Now what I don't understand is that the root node (which has a NULL parent_id) is not selected.
My understanding from the outer join is that it would return all items from the "left" tables regardless whether they have a corresponding row in the "right" table.
So given the data
name, id, parent_id
ROOT, 1, NULL
CHILD1, 2, 1
CHILD2, 3, 1
I would have expected the following result:
ROOT, NULL
ROOT, CHILD1
ROOT, CHILD2
but the row with (ROOT,NULL) is not returned.
I'm sure I'm missing something very obvious, but what?
Thanks in advance
Thomas
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