Virtual columns:
create table sales(sales_amt number,sales_category varchar(6)
generated always as (
case when sales_amt<=1000 then 'LOW'
else 'HIGH'
end
) virtual
);
SQL> desc sales;
Name
Null? Type
----------------------------------------- -------- -------------------SALES_AMT
SALES_CATEGORY
NUMBER
VARCHAR2(6)
SQL> select * from sales;
no rows selected
SQL> insert into sales values(3456);
insert into sales values(3456)
*
ERROR at line 1:
ORA-00947: not enough values
SQL> insert into sales(sales_amt) values(2345);
1 row created.
SQL> insert into sales(sales_amt) values(345);
1 row created.
SQL> select * from sales;
SALES_AMT
-----------------2345
345
SALE
------HIGH
LOW
Practise 2: Adding a virtual column to an existing table
SQL>create table virt_tab2 (name varchar2(10),marks1 number,marks2 number,marks3
number);
Table created.
SQL>insert into virt_tab2 values('venkat',34,23,56);
1 row created.
SQL>insert into virt_tab2 values('Raj',94,83,36);
1 row created.
SQL> alter table virt_tab2 add marks4 number;
// adding a normal column
Table altered.
SQL> select * from virt_tab2;
NAME
---------venkat
Raj
MARKS1
---------34
94
MARKS2
---------23
83
MARKS3
---------56
36
MARKS4
----------
SQL> alter table virt_tab2 add total number generated always as
(marks1+marks2+marks3+marks4) virtual;
//adding a virtual column
Table altered.
SQL> insert into virt_tab2(name, marks1,marks2,marks3,marks4) values('kiran',23,
12,78,90);
1 row created.
SQL> insert into virt_tab2(name, marks1,marks2,marks3,marks4)
values('venu',90,22,78,90);
1 row created.
SQL>select * from virt_tab2;
NAME
---------venkat
Raj
kiran
venu
MARKS1
---------34
94
23
90
MARKS2
---------23
83
12
22
MARKS3
---------56
36
78
78
MARKS4
----------
TOTAL
----------
90
90
203
280
Notes:
You can create indexes, partitions on a virtual columns
You can’t insert a value into the virtual column (through INSERT command)
The difference is that the virtual column is not stored, even though it can be accesses like
a normal column.
To prove that the virtual column is actually not taking
any space
Practice 3:
SQL>select * from virt_tab2;
NAME
---------venkat
Raj
kiran
venu
MARKS1
---------34
94
23
90
MARKS2
---------23
83
12
22
MARKS3
---------56
36
78
78
MARKS4
----------
TOTAL
----------
90
90
203
280
SQL>analyze table virt_tab2 compute statistics
Table analyzed.
SQL> select num_rows*avg_row_len from user_tables where table_name='VIRT_TAB2'
NUM_ROWS*AVG_ROW_LEN
---------------------------------------76 bytes
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------SQL> select * from virt_tab3;
NAME
MARKS1
MARKS2
MARKS3
MARKS4
---------- ---------- ---------- ---------- ---------venkat
34
23
56
Raj
94
83
36
kiran
23
12
78
90
venu
90
22
78
90
SQL> analyze table virt_tab3 compute statistics;
Table analyzed.
SQL>
SQL>
SQL> select num_rows*avg_row_len from user_tables where table_name='VIRT_TAB3'
NUM_ROWS*AVG_ROW_LEN
-------------------76 bytes
Even if you exclude the virtual column, the space is same. That means, the virtual
column is not actually taking any space.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------To check it in another way.
SQL> create table virt_tab5 as (select * from virt_tab2);
Table created.
SQL> analyze table virt_tab5 compute statistics;
Table analyzed.
SQL> select * from virt_tab5;
NAME
---------venkat
Raj
kiran
venu
MARKS1
---------34
94
23
90
MARKS2
---------23
83
12
22
MARKS3
---------56
36
78
78
MARKS4
----------
TOTAL
----------
90
90
203
280
SQL> select num_rows*avg_row_len from user_tables where table_name='VIRT_TAB5'
NUM_ROWS*AVG_ROW_LEN
---------------------------------------84 bytes
If another table with the same content, except a real column in place of the virtual
column, it’s taking 84 bytes, which is 8 bytes more than the original table.