Selecting from a sub-query

postgres=#
postgres=# CREATE TABLE employee (
postgres(# ID int,
postgres(# name varchar(10),
postgres(# salary real,
postgres(# start_date date,
postgres(# city varchar(10),
postgres(# region char(1)
postgres(# );
CREATE TABLE
postgres=#
postgres=# insert into employee (ID, name, salary, start_date, city, region)
postgres-# values (1, 'Jason', 40420, '02/01/94', 'New York', 'W');
INSERT 0 1
postgres=# insert into employee (ID, name, salary, start_date, city, region)
postgres-# values (2, 'Robert',14420, '01/02/95', 'Vancouver','N');
INSERT 0 1
postgres=# insert into employee (ID, name, salary, start_date, city, region)
postgres-# values (3, 'Celia', 24020, '12/03/96', 'Toronto', 'W');
INSERT 0 1
postgres=# insert into employee (ID, name, salary, start_date, city, region)
postgres-# values (4, 'Linda', 40620, '11/04/97', 'New York', 'N');
INSERT 0 1
postgres=# insert into employee (ID, name, salary, start_date, city, region)
postgres-# values (5, 'David', 80026, '10/05/98', 'Vancouver','W');
INSERT 0 1
postgres=# insert into employee (ID, name, salary, start_date, city, region)
postgres-# values (6, 'James', 70060, '09/06/99', 'Toronto', 'N');
INSERT 0 1
postgres=# insert into employee (ID, name, salary, start_date, city, region)
postgres-# values (7, 'Alison',90620, '08/07/00', 'New York', 'W');
INSERT 0 1
postgres=# insert into employee (ID, name, salary, start_date, city, region)
postgres-# values (8, 'Chris', 26020, '07/08/01', 'Vancouver','N');
INSERT 0 1
postgres=# insert into employee (ID, name, salary, start_date, city, region)
postgres-# values (9, 'Mary', 60020, '06/09/02', 'Toronto', 'W');
INSERT 0 1
postgres=#
postgres=# select * from employee;
id | name | salary | start_date | city | region
----+--------+--------+------------+-----------+--------
1 | Jason | 40420 | 1994-02-01 | New York | W
2 | Robert | 14420 | 1995-01-02 | Vancouver | N
3 | Celia | 24020 | 1996-12-03 | Toronto | W
4 | Linda | 40620 | 1997-11-04 | New York | N
5 | David | 80026 | 1998-10-05 | Vancouver | W
6 | James | 70060 | 1999-09-06 | Toronto | N
7 | Alison | 90620 | 2000-08-07 | New York | W
8 | Chris | 26020 | 2001-07-08 | Vancouver | N
9 | Mary | 60020 | 2002-06-09 | Toronto | W
(9 rows)

postgres=#
postgres=# -- Selecting from a sub-query
postgres=#
postgres=# SELECT 'test' AS test, id
postgres-# FROM (SELECT id FROM employee)
postgres-# AS example_sub_query;
test | id
------+----
test | 1
test | 2
test | 3
test | 4
test | 5
test | 6
test | 7
test | 8
test | 9
(9 rows)

postgres=#
postgres=# drop table employee;
DROP TABLE
postgres=#
postgres=#
postgres=#