Insert unique values after addition of a column in a table - sql

Question

We have a table with two columns and have added another column recently (named sequence_no) , Is there a way to insert unique values like , 1,2,3 for every row in the table ?
eg
table name : test
desc test
name varchar2
value varchar2
--> n_seq_no number
select * from test
Name value n_Seq_no
test1 100
test2 200
test3 300
test4 500
The table already had name, and value as the columns of the table, I need to add unique values for the n_Seq_no column with the existing data,
Output format:
select * from test
Name value n_Seq_no
test1 100 1
test2 200 2
test3 300 3
test4 500 4
and so on for all the rows in table.

Answer 1

You could simply set the new column as ROWNUM.
Something like,
SQL> CREATE TABLE t(
2 A NUMBER,
3 b NUMBER);
Table created.
SQL>
SQL> INSERT INTO t(A) VALUES(100);
1 row created.
SQL> INSERT INTO t(A) VALUES(200);
1 row created.
SQL> INSERT INTO t(A) VALUES(300);
1 row created.
SQL>
SQL> SELECT * FROM t;
A B
---------- ----------
100
200
300
SQL>
SQL> UPDATE t SET b = ROWNUM;
3 rows updated.
SQL> SELECT * FROM T;
A B
---------- ----------
100 1
200 2
300 3
SQL>
If you are on 12c, you could use an IDENTITY COLUMN.

Answer 2

Assuming that your table is really big it's better to recreate and repopulate:
rename test to old_test;
create table new_test
as
select t.*, rownum as n_seq_no
from old_test t
order by value;
Don't forget to migrate grants, indexes, triggers and etc if any.
UPDATE: ordering is optional. It is required only if you want to assign n_seq_no value using some predefine ordering.