How to add ‘ON DELETE CASCADE’ in ALTER TABLE statement

I have a foreign key constraint in my table, I want to add ON DELETE CASCADE to it.

I have tried this:

alter table child_table_name
  modify constraint fk_name
  foreign key (child_column_name)
  references parent_table_name (parent_column_name) on delete cascade;

Doesn’t work.

EDIT:
Foreign key already exists, there are data in foreign key column.

The error message I get after executing the statement:

ORA-02275: such a referential constraint already exists in the table

Answer

You can not add ON DELETE CASCADE to an already existing constraint. You will have to drop and re-create the constraint. The documentation shows that the MODIFY CONSTRAINT clause can only modify the state of a constraint (i-e: ENABLED/DISABLED…).

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Answer