使用外键约束从MySQL表中删除

Question

I have two tables, with some data loaded in it and the two reference each other. Table B references the primary key of Table A. I manually attempt to delete some table rows from Table A that are present in Table B, and I get this:

#1451 - Cannot delete or update a parent row: a foreign key constraint fails ( TableA . TableB , CONSTRAINT TableB_ibfk_2 FOREIGN KEY ( column ) REFERENCES flashcard ( primaryKeyColumn ))

I am not quite sure what's up here. From what I understand,if I delete some rows of TableA then they should automatically also delete the corresponding rows present in Table B (ie if they are present in Table B) but clearly this isn't the case here and it's prompting the above error.

Anybody wanna whack the side of my head and let me know why ?

Answer 1

Unfortunately, the automatic deletion that you think should happen does not happen. You would need to manually delete the rows in Table B that contain the ID from the row in Table A that you wish to remove.

Try adding ON DELETE CASCADE to your foreign key column definition in Table B.

If you are using Hibernate, or some other ORM tool, it would have a "cascade" mechanism that would handle this automatically. But here since you are interfacing with the database directly, you must keep in mind that a row cannot be deleted if the ID of said row exists in any other table as a foreign key (as in your case, the primary key of A exists in some of the rows in B). Unless of course you specify the ON DELETE CASCADE property.