Self Joins Using SQL/92

CREATE TABLE departments
(department_id             number(10)            not null,
 department_name           varchar2(50)      not null,
 CONSTRAINT departments_pk PRIMARY KEY (department_id)
);

insert into departments ( department_id, department_name )values(1,'Data Group' );
insert into departments ( department_id, department_name )values(2,'Purchasing' );
insert into departments ( department_id, department_name )values(3,'Call Center' );
insert into departments ( department_id, department_name )values(4,'Communication' );

CREATE TABLE employees
( employee_id          number(10)      not null,
  last_name            varchar2(50)      not null,
  job_id               varchar2(30),
  department_id        number(10),
  salary               number(6),
  manager_id           number(6),
  CONSTRAINT           employees_pk PRIMARY KEY (employee_id),
  CONSTRAINT           fk_departments FOREIGN KEY (department_id) 
                       REFERENCES departments(department_id)
);
insert into employees( employee_id, last_name, job_id, salary,department_id ,manager_id)
values(                1001,        'Lawson',  'MGR',  30000, 1,             1004);
insert into employees( employee_id, last_name, job_id, salary, department_id ,manager_id)
values(                1002,        'Wells',   'DBA',  20000, 2,             1005 );
insert into employees( employee_id, last_name, job_id, salary, department_id ,manager_id)
values(                1003,        'Bliss',   'PROG', 24000, 3,             1004);
insert into employees( employee_id, last_name, job_id, salary, department_id, manager_id)
values(                1004,        'Kyte',    'MGR',  25000 ,4,             1005);
insert into employees( employee_id, last_name, job_id, salary, department_id, manager_id)
values(                1005,        'Viper',   'PROG', 20000, 1,             1006);
insert into employees( employee_id, last_name, job_id, salary, department_id,manager_id)
values(                1006,        'Beck',    'PROG', 20000, 2,             null);
insert into employees( employee_id, last_name, job_id, salary, department_id, manager_id)
values(                1007,        'Java',    'PROG', 20000, 3,             1006);
insert into employees( employee_id, last_name, job_id, salary, department_id, manager_id)
values(                1008,        'Oracle',  'DBA',  20000, 4,             1006);


SQL> SELECT w.last_name
  2  FROM employees w, employees m
  3  WHERE w.manager_id = m.employee_id;

LAST_NAME
--------------------------------------------------
Lawson
Wells
Bliss
Kyte
Viper
Java
Oracle

7 rows selected.

SQL>

SQL> SELECT w.last_name
  2  FROM employees w INNER JOIN employees m
  3  ON w.manager_id = m.employee_id;

LAST_NAME
--------------------------------------------------
Lawson
Wells
Bliss
Kyte
Viper
Java
Oracle

7 rows selected.

SQL>