How does Oracle determine a table as key-preserved or not?

Question

I tried joining employees and departments in HR schema. Normally, departments is not key-preserved in the join operation. But I've arranged in the view so that each department has exactly one employee, so that dept_no may become the key for the join. But still, it said "cannot modify non key-preserved table". Any hints? does the joining type (left or right or inner or outer) affect the mechanism on how Oracle determine which are key-preserved and which are not? thanks.

Answer 1

Hi,
You can achive in many ways... demo
Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.
C:\Users\Pavan>sqlplus scott/tiger@orcl
SQL*Plus: Release 11.2.0.1.0 Production on Sun Dec 19 14:19:36 2010
Copyright (c) 1982, 2010, Oracle.  All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> create table t_parent(
  2  code varchar2(10)
  3  ,description varchar2(50)
  4  )
  5  ;
Table created.
SQL> create table t_detail(
  2  the_code varchar2(10)
  3  ,the_date date
  4  );
Table created.
SQL> insert into t_parent values('a','first letter');
1 row created.
SQL> insert into t_detail values ('a',sysdate);
1 row created.
SQL> commit;
Commit complete.
SQL> select * from t_parent;
CODE       DESCRIPTION
a          first letter
SQL> select * from t_detail;
THE_CODE   THE_DATE
a          19-DEC-10
SQL> select * from t_parent join t_detail on the_code=code;
CODE       DESCRIPTION                                        THE_CODE
THE_DATE
a          first letter                                       a
19-DEC-10
SQL> create or replace view test_v
  2  as
  3  select *
  4  from t_parent join t_detail on the_code=code;
View created.
SQL> select * from test_v;
CODE       DESCRIPTION                                        THE_CODE
THE_DATE
a          first letter                                       a
19-DEC-10
SQL> update test_v set description='x';
update test_v set description='x'
ERROR at line 1:
ORA-01779: cannot modify a column which maps to a non key-preserved table
SQL> create or replace trigger trig1
  2   instead of update on test_v
  3      for each row
  4      begin
  5      if :old.description <> :new.description then
  6      update t_parent
  7      set description = :new.description;
  8    end if;
  9   end;
10  /
Trigger created.
SQL> update test_v set description='x';
1 row updated.
SQL> commit;
Commit complete.
SQL> select * from test_v;
CODE       DESCRIPTION                                        THE_CODE
THE_DATE
a          x                                                  a
19-DEC-10- Pavan Kumar N