adding a primary/unique key to existing table and updating values

lets take a simple example here. create a table and leave id column empty and add values to other column name.

Sample create table :

create table abc (id number, name varchar2(20));

add values to name column and leave the id column empty

INSERT INTO ABC (NAME) VALUES ('a');

INSERT INTO ABC (NAME) VALUES ('b');

INSERT INTO ABC (NAME) VALUES ('c');

INSERT INTO ABC (NAME) VALUES ('d');

INSERT INTO ABC (NAME) VALUES ('e');

INSERT INTO ABC (NAME) VALUES ('f');

INSERT INTO ABC (NAME) VALUES ('g');

INSERT INTO ABC (NAME) VALUES ('h');

Now we have a situation where we have a table with only values in the name column and id column is empty (in case if u want to add id's to already existing table  just add a column for the new unique ID's ). 

Now we will create a sequence to get the sequential values to update with.

create a sequence :

CREATE SEQUENCE SEQ_abc

START WITH 1

MAXVALUE 99999

MINVALUE 1

NOCYCLE

NOCACHE

NOORDER;

This cursor will update the ID's columns with the new unique ID's from above sequence sequentially. execute below block and all set.

Update cursor:

DECLARE

Cursor store_id

IS

SELECT id FROM abc FOR UPDATE;

BEGIN

FOR c_store_id IN store_id LOOP

UPDATE abc

SET id = SEQ_abc.nextval

WHERE CURRENT OF store_id;

END LOOP;

commit;

END;

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Note: This is a one time update, use a trigger  to keep column updated everytime.