MySQL : add constraint gives:ERROR 1452 (23000): Cannot add or update a child row?

You asked mysql to enforce a relationship between the two tables which it was previously unaware of. Unfortunately the state of the two tables indicates that the relationship is already broken. That's just bad luck.

Now you have some records in the rc_sent_items_table with profile_ids that don't exist in the rc_profile_table.

Check that field rc_sent_items_table. Profile_id_to has appropriate values in accordance with rc_profile_table.id.

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