This is a bad idea. There are two ways to do this,

1. ltree
2. hierarchical single table

Using the hierarchical single table is the best fit for you. It's a minor re-organization but it's much more semantic.

CREATE TABLE teams (
  id      serial PRIMARY KEY,
  parent  int    REFERENCES teams,
  name    text
);

INSERT INTO teams (id, parent, name) VALUES
  ( 1, null, 'A'  ),
  ( 2, null, 'B'  ),
  ( 3, 2,    'B1' ),
  ( 4, 2,    'B2' );

For an example of a recursive query for this..

WITH RECURSIVE t(id,name,parent) AS (
  SELECT t1.id, t1.name, ARRAY[]::text[]
  FROM teams AS t1
  WHERE parent IS NULL
  UNION ALL
    SELECT t2.id, t2.name, t1.parent || ARRAY[t1.name]
    FROM t AS t1
    JOIN teams AS t2
    ON t2.parent = t1.id
)
SELECT *
FROM t;

 id | name | parent 
----+------+--------
  1 | A    | {}
  2 | B    | {}
  3 | B1   | {B}
  4 | B2   | {B}
(4 rows)

This allows arbitrarily deep hierarchy.

INSERT INTO teams (id, parent, name) VALUES
  ( 5, 4, 'Deep' );

Running the same query as above,

 id | name | parent 
----+------+--------
  1 | A    | {}
  2 | B    | {}
  3 | B1   | {B}
  4 | B2   | {B}
  5 | Deep | {B,B2}
(5 rows)

One way is use left join, and join teams twice:

select
    coalesce(t1.name, '-') "Team Name", coalesce(t2.name, '-') "Parent Name"
from team_mapping tm
left join teams t1 on tm.team_id = t1.id
left join teams t2 on tm.parentid = t2.id

then you can use subquery in select statement:

select
    coalesce((select t.name from teams t where t.id = tm.team_id), '-') "Team Name",
    coalesce((select t.name from teams t where t.id = tm.parentid), '-') "Parent Name"
from team_mapping tm

Edit: Previous first answer's join is incorrect, it should be left join in case of there is no parentid in table teams. Besides, for null s, use coalesce to convert to -.

Demo in sqlfiddle.