I can't delete row after append it a foreign key

问题

I have two table second and third

second_id(PRIMARY KEY)  second_name           
   1                    .........
   2                    .......
   3                    .........

third_id(PRIMARY KEY) third_name second_id(FOREIGN KEY for second.second_id)
      1                  .....                 1
      2                  .....                 1
      3                  .....                 1
      4                  .....                 2
      5                  .....                 2

Now i want to delete a row fromsecond where second_id=2 (DELETE FROM second WHERE second_id=2) but it does not work. It says Successful 0 row(s) affected What is more is, it happened after i append a foreign key to third.second_id (i added foreign key after table is created).

回答1:

You will have to delete the rows in third table with second_id = 2 and then delete the rows from second with id = 2 or have ON DELETE CASCADE in the foreign key constraint

回答2:

You can't delete a row from second because one of it's values is being used in third. That's a Foreign Key rule.

But you can cascade the foreign key in a was that if you delete a row from second, all rows in third which uses this value will also be deleted, or you can set them to null.

This manual from MYSQL can tell you how to properly use ON DELETE and ON UPDATE, and then you can pick up the best option for you. Look up for Referential Actions

Edit:

You can't add ON DELETE CASCADE to an existing foreign key, so you will have to drop it first.

alter table third drop foreign key third_foreign

Then

alter table third add constraint third_foreign foreign key (second_id) references second(second_id) on delete cascade;

来源：https://stackoverflow.com/questions/34501590/i-cant-delete-row-after-append-it-a-foreign-key

标签

mysql

foreign-keys

foreign-key-relationship