MySQL：在具有多个外键依赖项的表上更新

Question

Below is the simplified picture of the relationships I have in my DB:

create table attribute (id int auto_increment, primary key (id));
create table state_sample (id int auto_increment, primary key(id));
create table state_sample_attribute (
    state_sample_id int,
    attribute_id int,
    primary key(state_sample_id, attribute_id),
    foreign key (state_sample_id) references state_sample(id) on update cascade,
    foreign key (attribute_id) references attribute(id) on update cascade
);
create table note (
    id int auto_increment,
    state_sample_id int,
    attribute_id int,
    primary key(id),
    foreign key (state_sample_id) references state_sample(id) on update cascade,
    foreign key (state_sample_id, attribute_id)
        references state_sample_attribute(state_sample_id, attribute_id) on update cascade
);

insert into attribute values (1);
insert into state_sample values (1);
insert into state_sample_attribute values (1, 1);
insert into note values (1, 1, 1);

Whenever I try to update the ss table, it fails:

update state_sample set id = 2;

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`demotemplate`.`note`, CONSTRAINT `note_ibfk_1` FOREIGN KEY (`ss_id`) REFERENCES `ss` (`id`) ON UPDATE CASCADE)

As far as I understand, this is what happens:

• It tries to set state_sample.id = 2.
• It sees the cascade to note and tries to update note.state_sample_id.
• However, note.state_sample_id is also involved in the foreign key to to state_sample_attribute(state_sample_id, attribute_id), so it goes to check whether that's still valid.
• As state_sample_attribute.state_sample_id has not yet been updated, the constraint fails.

Is my assumption correct? And if so, is there a way to work this around?

Answer 1

Give the ssa table its own id primary key, and use that in the foreign key in notes , rather than referencing the ss_id and a_id columns.

create table ssa (
    id int auto_increment,
    ss_id int, 
    a_id int, 
    primary key (id),
    unique key (ss_id, a_id), 
    foreign key (ss_id) references ss(id) on update cascade, 
    foreign key (a_id) references a(id) on update cascade);
create table note (
    id int auto_increment, 
    ss_id int, 
    ssa_id int, 
    primary key(id), 
    foreign key (ss_id) references ss(id) on update cascade, 
    foreign key (ssa_id) references ssa(id) on update cascade);

Now you don't have the redundant dependency.

It's also not clear that note needs ss_id at all, since it's redundant with the related ssa row.

Answer 2

Try

DISABLE KEYS

or

SET FOREIGN_KEY_CHECKS=0;

make sure to turn it on

SET FOREIGN_KEY_CHECKS=1;

after.

Answer 3

最终解决问题的是放弃了额外的FK：

alter table note drop foreign key (state_sample_id) references state_sample(id);