Key preserved table

What does mean by key preserved table?

According to its definition: A table is key-preserved if every key of the table can also be a key of the result of the join. So, a key-preserved table has its keys preserved through a join.

Let's illustrate this by an example.

 
create table a (a_id number primary key);
create table b (b_id number primary key, a_id number,
  foreign key (a_id) REFERENCES a(a_id) on delete cascade);

create or replace view c as (
  select a.a_id, b.b_id from a,b where a.a_id = b.a_id
)

insert into a values (1);
insert into b values (2,1);
insert into b values (3,1);

select * from c;

The result is

 
A_ID B_ID
---------
1    2
1    3

Clearly A's primary key A_ID cannot be used as a primary key in the view, so table A is not a key preserved table. On the other hand, table B's primary key B_ID can be used as a primary key in the view, so table B is a key preserved table for this view C.

So what happens when you

 
delete from c;

is all the rows in table B are deleted.

Rules for updating a join view

http://docs.oracle.com/cd/E11882_01/server.112/e25494/views.htm#ADMIN11782