Question

I have a table denoting parent-child relations. The relations can go n-level deep.

I have created a sample table using the following query:

CREATE SEQUENCE relations_rel_id_seq
    INCREMENT BY 1
    NO MAXVALUE
    NO MINVALUE
    CACHE 1;
CREATE TABLE relations(
    rel_id bigint DEFAULT nextval('relations_rel_id_seq'::regclass) NOT NULL PRIMARY KEY,
    rel_name text,
    rel_display text,
    rel_parent bigint
);

SQLFiddle

I need to query the table and display the parent-child relations hierarchically. I’m still not getting an idea regarding how to query n-level deep using sql query.

For the sqlfiddle eg, the expected hierarchy of output:

rel1
    rel11
        rel111
        rel112
            rel1121
rel2
    rel21
        rel211
        rel212

N.B: The value n, in n-level is unknown.

DB Design:

Is there any better way such a relation can be expressed in the
database for easy querying.?

Answer 1

With Postgres you can use a recursive common table expression:

with recursive rel_tree as (
   select rel_id, rel_name, rel_parent, 1 as level, array[rel_id] as path_info
   from relations 
   where rel_parent is null
   union all
   select c.rel_id, rpad(' ', p.level * 2) || c.rel_name, c.rel_parent, p.level + 1, p.path_info||c.rel_id
   from relations c
     join rel_tree p on c.rel_parent = p.rel_id
)
select rel_id, rel_name
from rel_tree
order by path_info;

SQLFiddle based on your example: http://sqlfiddle.com/#!11/59319/19

(I replaced the spaces for indention with underscores as SQLFiddle doesn’t display the spaces correctly)