在mysql表中实现外键

Question

Im trying to restore some old database tables, that when i build them i did not user foreign keys. I have the field that corresponds to the Foreign Key, but i've not set it up in the relations table to which table it connected.

Right know i have a problem because if i try to add that relation, it cannot, because there are some rows deleted in the other table.

Is there any mysql command for checking this type NULL relations for me to delete the rows that i dont need.. and in the end.. add the relation.

TableA
    id,
    name

TableB
   id,
   tableA_id,
   points

I've deleted some TableA rows.. now i cannot had that relation.

Any mysql command to help, or need to check manually?

Thanks

Answer 1

Assuming that you have PRIMARY KEY constraint at least on TableA.id you can try

-- Delete all orphaned records from TableB
DELETE b
  FROM tableb b LEFT JOIN tablea a
    ON b.a_id = a.id
 WHERE a.id IS NULL;
-- Create a FK constraint 
ALTER TABLE TableB 
ADD CONSTRAINT fk_a_id FOREIGN KEY (a_id) REFERENCES tablea(id);

Here is SQLFiddle demo