sql trees move some element

I have a table:
id | integer | not null default
nextval('public.menu_2_id_seq'::text)
parent_id | integer |
description | text | not null

how am I able to move some element with id = x before or after
element with id = y ?

In relational databases there is no such concept of "before"
and "after". If you want to make id.x smaller then id.y you
can just update the column id to the values you desire. You
then need to make sure that you don't get in the way of the id
value of other rows. One way to do it would be to compare the
values of id.x and id.y and switch them around if necessary.

Karsten
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