DBA Tips Archive for Oracle


Locating duplicate rows when trying to create UNIQUE INDEX

by Jeff Hunter, Sr. Database Administrator

Suppose you are trying to create a UNIQUE index on a column and get the following error:

   SQL> desc emp
    Name                            Null?    Type
    ------------------------------- -------- ----
    EMPNO                                    NUMBER(15)
    NAME                                     VARCHAR2(20)
    DEPTNO                                   NUMBER(15)

   SQL> CREATE UNIQUE INDEX emp_i ON emp (empno);
   CREATE UNIQUE INDEX emp_i ON emp (empno)
   ERROR at line 1:
   ORA-01452: cannot CREATE UNIQUE INDEX; duplicate keys found
Oracle is reporting that it cannot create a UNIQUE INDEX on the table because it found duplicate records while trying to create the index. You know have to find the duplicate values. This month I provide three methods for locating duplicate rows in a table.

Method #1 : Using a self join
    , empno
    , name
  FROM emp a
  WHERE a.rowid > (SELECT MIN(b.rowid)
                   FROM emp b
                   WHERE b.empno = a.empno);

  ROWID                   EMPNO NAME
  ------------------ ---------- --------------------
  0000059F.0001.0001        100 Jeff
Method #2 : Using a GROUP BY
    , empno
  FROM emp
  GROUP BY empno
  HAVING COUNT(*) > 1;

    COUNT(*)      EMPNO
  ---------- ----------
           2        100
Method #3 : Using the "into exceptions" clause
  EXCEPTIONS INTO exceptions;

  alter table emp
  ERROR at line 1:
  ORA-02299: cannot enable (OPS$ORACLE.EMP_I) - duplicate keys found

  -- NOW JOIN THE exceptions TABLE TO THE emp TABLE BY rowid

    , a.name
      emp a
    , exceptions b
  WHERE a.rowid = b.row_id;

  ---------- --------------------
         100 Mark
         100 Jeff

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Last modified on
Wednesday, 21-Mar-2001 00:00:00 EST
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