How to add primary key for table with existing data?

Question

The table is occupied data already. There was no primary key before, so for every column, there are some duplicate values.
I want to add a new column, which should be of the datatype integer, and can automatically increment, starting from 001. I tried this in Oracle SQL Developer, but it says "ORA-01758: table must be empty to add mandatory (NOT NULL) column". How can I do it? Thanks!

Answer 1

Hello, don't call the column ID, that could be an ID relating to anything. Perhaps be a little more specific like emp_id .
Then to update it:
UPDATE Employee emp
  SET emp_id = (
  SELECT row_num
    FROM (
  SELECT ROWID,
             ROW_NUMBER() OVER (ORDER BY ROWID) row_num
    FROM Employee emp2) emp2
  WHERE emp.ROWID = emp2.ROWID);That will assign a new EMP_ID for each, beginning at 1.
Then:
ALTER TABLE Employee MODIFY emp_id NOT NULL;
ALTER TABLE Employee ADD CONSTRAINT EMP_PK PRIMARY KEY (EMP_ID);Then create a sequence for future population of the Employee table:
CREATE SEQUENCE emp_seq START WITH <maximum value of emp_id + 1>;