How to find duplicate data

Question

Dear All,
please help me.
I have a table named OT it has two columns which i want to apply composite primary key. but when i apply it returns error ORA-02437: cannot vlidate - primary key voilated, i applied enable novalidate but error still there. please let me know how can i find where is and what is duplicate data.
please reply thank you.....

Answer 1

Isn't this easy to understand?
create table test_table
  col1        number,
  col2        number,
  col         varchar2(1)
insert into test_table values (1, 1, 'A');
insert into test_table values (1, 1, 'B');
insert into test_table values (1, 1, 'C');
insert into test_table values (1, 1, 'D');
insert into test_table values (1, 2, 'D');
insert into test_table values (1, 3, 'D');
insert into test_table values (1, 4, 'D');
select *
  from test_table;
COL1                   COL2                   COL
1                      1                      A  
1                      1                      B  
1                      1                      C  
1                      1                      D  
1                      2                      D  
1                      3                      D  
1                      4                      D  
7 rows selected
delete
  from test_table a
where rowid < (
                  select max(rowid)
                    from test_table b
                   where a.col1 = b.col1
                     and a.col2 = b.col2
select *
  from test_table;
COL1                   COL2                   COL
1                      1                      D  
1                      2                      D  
1                      3                      D  
1                      4                      D