On delete cascade - where I have to add it

问题

I needed an example of cascade delete. My question is, where do I add it? In the table I create PK, or in other tables where this PK is sitting there as FK?

Can I use "alter table" to add "on delete cascade" to existing table? An example please?

@edit MYSQL, using phpMyAdmin

@edit 2

Does it look good?

alter table wplaty
  drop foreign key pesel,
  add constraint pesel foreign key (pesel)
    references baza_osob(pesel) on delete cascade on update restrict;

My parent table = baza_osob My child table = wplaty

PK is pesel, FK is pesel as well.

@edit3 Got error:

#1025 - Error on rename of '.\projekt\wplaty' to '.\projekt#sql2-1300-6c' (errno: 152)

回答1:

cascade directives go into the "child" tables, e.g.

create table parent (
   id int primary key
)

create table child (
   id int primary key
   parent_id int,
   foreign key (parent_id) references parent (id)
      on delete cascade
)

Never tried doing an alter on a foreign key to change its on settings, but worst case, you simply drop the existing FK and redefine it with the new on settings in place.

回答2:

You actually don't add the 'ON DELETE CASCADE' on a table. It's part (or not) of each Foreign Key definition.

回答3:

You need to enable this option for the foreign key. If you did not add this option when you created foreign key, then you should recreate it.

alter table <table> drop foreign key <fk name>;

alter table <table> add constraint <fk name> foreign key (<column name>)
  references <ref tble>(<ref column) on delete cascade on update restrict;

In one statement:

alter table <table>
  drop foreign key <old fk name>,
  add constraint <new fk name> foreign key (<column name>)
    references <ref tble>(<ref column) on delete cascade on update restrict;

来源：https://stackoverflow.com/questions/13920078/on-delete-cascade-where-i-have-to-add-it

标签

php

mysql

sql

Cascade

cascading-deletes