title: How to add 'ON DELETE CASCADE' in ALTER TABLE statement
body: <p>I have a foreign key constraint in my table, I want to add ON DELETE CASCADE to it.</p>
  
  <p>I have tried this:</p>
  
  <pre>
  alter table child_table_name
    modify constraint fk_name
    foreign key (child_column_name)
    references parent_table_name (parent_column_name) on delete cascade;
  </pre>
  
  <p>Doesn't work.</p>
  
  <p>EDIT:<br>
  Foreign key already exists, there are data in foreign key column.</p>
  
  <p>The error message I get after executing the statement:</p>
  
  <pre>
  ORA-02275: such a referential constraint already exists in the table
  </pre>