Is it possible to create a view where table in the From clause changes name

Question

is it possible to create a view from a from a table like this
create view my_view as select id, col1, col2, result from <<my_latest_cacahe_table>>;
the table in the from clause changes the name .
I have another table which indicates the the latest name of my cache tables. Always there are two records there. The latest one and previous one.
select * from cache_table_def
table_name cache_table_name refresh_date
my_table cache_table245 1/23/2012
my_table cache_table235 1/22/2012
create table cache_table_def (table_name varchar2(25), cache_table_name varchar2(25), refresh_date date);
insert into cache_table_def values( 'my_table','cache_table245','23-jan-2012');
insert into cache_table_def values ( 'my_table','cache_table546','22-jan-2012');
create table cache_table245 (id number, col1 varchar2(50), col2 varchar2(20), result number);
insert into cache_table245 values(1, 'test123', 'test345',12.12);
insert into cache_table245 values (2, 'test223', 'test245',112.12);
create table cache_table235 (id number, col1 varchar2(50), col2 varchar2(20), result number);
insert into cache_table235 values (1, 'test123', 'test345',92.12);
insert into cache_table235 values (2, 'test223', 'test245',222.12);
what I need to do is find the latest cache_table name for my_table and use that in my view defintion
When user select from the the view it always reurns the data from the latest cache_table
is it possible to do something like this in oracle 11g?
I have no control on the cache tables names. that is why I need to use the latest name from the table.
Any ideas really appreciated.

Answer 1

I've worked up an example that does what you ask. It uses the SCOTT schema EMP table. Make two copies of the EMP table, EMP1 and EMP2. I deleted dept 20 from emp1 and deleted dept 30 from emp2 so I could see that the result set really came from a different table.
-- create a context to hold an environment variable - this will be the table name we want the view to query from
create or replace context VIEW_CTX using SET_VIEW_FLAG;
-- create the procedure specified for the context
- we will pass in the name of the table to query from
create or replace procedure SET_VIEW_FLAG ( p_table_name in varchar2 default 'EMP')
  as
  begin
      dbms_session.set_context( 'VIEW_CTX', 'TABLE_NAME', upper(p_table_name));
  end;
-- these are the three queries - one for each table - none of them will return data until you set the context variable.
select * from emp where 'EMP' = sys_context( 'VIEW_CTX', 'TABLE_NAME' );
select * from emp1 where 'EMP1' = sys_context( 'VIEW_CTX', 'TABLE_NAME' );
select * from emp2 where 'EMP2' =  sys_context( 'VIEW_CTX', 'TABLE_NAME' )
-- this is how you set the context variable depending on the table you want the view to query
exec set_view_flag( p_table_name => 'EMP' );
exec set_view_flag( p_table_name => 'EMP1' );
exec set_view_flag( p_table_name => 'EMP2');
-- this will show you the current value of the context variable
SELECT sys_context( 'VIEW_CTX', 'TABLE_NAME' ) FROM DUAL
-- this is the view definition - it does a UNION ALL of the three queries but only one will actually return data
CREATE VIEW THREE_TABLE_EMP_VIEW AS
select * from emp where 'EMP' = sys_context( 'VIEW_CTX', 'TABLE_NAME' )
union all
select * from emp1 where 'EMP1' = sys_context( 'VIEW_CTX', 'TABLE_NAME' )
union all
select * from emp2 where 'EMP2' =  sys_context( 'VIEW_CTX', 'TABLE_NAME' )
-- first time - no data since context variable hasn't been set yet
SELECT * FROM THREE_TABLE_EMP_VIEW
-- get data from the EMP table
exec set_view_flag( p_table_name => 'EMP' );
SELECT * FROM THREE_TABLE_EMP_VIEW
-- get data from the EMP2 table
exec set_view_flag( p_table_name => 'EMP2');
SELECT * FROM THREE_TABLE_EMP_VIEW
For your use case you just have to call the context procedure whenever you want to switch tables. You can union all as many queries as you want and can even put WHERE clause conditions based on other filtering criteria if you want. I have used this approach with report views so that one view can be used to roll up report data different ways or for different regions, report periods (weekly, quarterly, etc). I usually use this in a stored procedure that returns a REF CURSOR to the client. The client requests a weekly report and provides a date, the procedure calculates the START/END date based on the one date provided and sets context variables that the view uses in the WHERE clause for filtering.
For reporting it works great!