create new columns and unselect existing columns without altering table schema - database

Question

new to PL/SQL and wondering if I'd be able to create new columns, drop existing columns WITHOUT altering the table schema/source table, so that the source table remains untouched while the procedure returns a new result set that has new columns. for example,
id name type
1 foo apple
2 bar orange
Is it possible to write a stored procedure that unselects the column 'type' and creates new column 'value' that satisfies following condition:
if name = 'foo' then value = 5
elsif name = 'bar' then value = 7
so my result set would look like this:
id name value
1 foo 5
2 bar 7
Can this be done by purely using the pl/sql engine?
Secondly, would it be possible to create a variable that stores the output of a SQL that generates a alter table statement, and execute that variable so the alter table statement gets executed?

Answer 1

Looks like you actually want a view, not any kind of PL/SQL code.
Table:
SQL> create table test as
2 select 1 id, 'foo' name, 'apple' type from dual union all
3 select 2, 'bar', 'orange' from dual;
Table created.
View:
SQL> create or replace view v_test as
2 select id,
3 name,
4 case when name = 'foo' then 5
5 when name = 'bar' then 7
6 end as value
7 from test;
View created.
Result:
SQL> select * from v_test;
ID NAM VALUE
---------- --- ----------
1 foo 5
2 bar 7
SQL>
If you really want PL/SQL, then using a function which returns ref cursor might be one option; you'd do that "logic" stuff in its select statement (just like in the view):
SQL> create or replace function f_test
2 return sys_refcursor
3 is
4 rc sys_refcursor;
5 begin
6 open rc for
7 select id,
8 name,
9 case when name = 'foo' then 5
10 when name = 'bar' then 7
11 end as value
12 from test;
13
14 return rc;
15 end;
16 /
Function created.
Testing:
SQL> select f_test from dual;
F_TEST
--------------------
CURSOR STATEMENT : 1
CURSOR STATEMENT : 1
ID NAM VALUE
---------- --- ----------
1 foo 5
2 bar 7
SQL>
As of your question:
... create new columns, drop existing columns WITHOUT altering the table schema/source table
Nope. That's done with ALTER TABLE.
Finally, as of your second question: yes, you can use a variable and execute it using dynamic SQL, i.e. execute immediate:
SQL> select * from test;
ID NAM TYPE
---------- --- ------
1 foo apple
2 bar orange
SQL> declare
2 l_str varchar2(200);
3 begin
4 l_str := 'alter table test rename column type to value';
5 execute immediate l_str;
6 end;
7 /
PL/SQL procedure successfully completed.
SQL> select * from test;
ID NAM VALUE --> column name has changed
---------- --- ------
1 foo apple
2 bar orange
SQL>