FK reference PK in same table

Question

Hello,
I have a table defined as follows
create table MATABLE  (
   ID                   INTEGER                         not null,
   FK                   INTEGER                        not null,
   constraint PK_MATABLE primary key (ID)
alter table MATABLE
   add constraint FK_MATABLE_REFERENCE_MATABLE foreign key (FK)
      references MATABLE (ID);*********************************************
I understand the FK reference the PK in the same table. My first question is can anybody give me a practical example of having the above architecture/relation. A sample of a few lines together with a short explanation of a business logic example would be great.

Answer 1

I am assuming that this is one of the posts that had the wrong date applied between September 3rd and 7th, 2004.
The following is an example of a practical application. In this example, using the Oracle emp demo table, a constraint is placed such that any manager (mgr)entered must already be an employee (empno). This prevents entry of a manager, who is not an employee.
scott@ORA92> -- emp data:
scott@ORA92> select empno, mgr, deptno from emp
  2  /
     EMPNO        MGR     DEPTNO
      7369       7902         20
      7499       7698         30
      7521       7698         30
      7566       7839         20
      7654       7698         30
      7698       7839         30
      7782       7839         10
      7788       7566         20
      7839                    10
      7844       7698         30
      7876       7788         20
      7900       7698         30
      7902       7566         20
      7934       7782         10
14 rows selected.
scott@ORA92> -- add constraint:
scott@ORA92> alter table emp
  2  add constraint emp_fk_emp foreign key (mgr)
  3  references emp (empno)
  4  /
Table altered.
scott@ORA92> -- will not allow insert of manager (mgr) who is not an employee (empno):
scott@ORA92> insert into emp (empno, mgr, deptno) values (1111, 9999, 10)
  2  /
insert into emp (empno, mgr, deptno) values (1111, 9999, 10)
ERROR at line 1:
ORA-02291: integrity constraint (SCOTT.EMP_FK_EMP) violated - parent key not found
scott@ORA92> -- will allow insert of manager (mgr) who is an employee (empno):
scott@ORA92> insert into emp (empno, mgr, deptno) values (1111, 7934, 10)
  2  /
1 row created.