title: ORA-02437: cannot validate <name> - primary key violated
body: <p>I have a table:</p>
  
  <pre><code>CREATE TABLE MY_TABLE (
    MY_ID NUMBER NOT NULL,
    COLUMN_1 NUMBER,
    COLUMN_2 NUMBER
  );
  ALTER TABLE MY_TABLE ADD CONSTRAINT PK_FOO PRIMARY KEY (MY_ID);
  </code></pre>
  
  <p>at a later point, when executing the following sql, I get an error:</p>
  
  <pre><code>ALTER TABLE MY_TABLE DROP PRIMARY KEY DROP INDEX;
  ALTER TABLE MY_TABLE ADD CONSTRAINT PK_FOO PRIMARY KEY (MY_ID) 
  
  ORA-02437: cannot validate PK_FOO - primary key violated
  </code></pre>
  
  <p>My table only contains 3 entries all with a different primary key which is also not null.
  Anyone has an idea what this could be?</p>
  
  <p>Thanks, </p>
  
  <p>Peter</p>