如果从子表中删除行，则“在删除级联上”

Question

I'm having a difficult time understanding "on delete cascade" If I had the following example:

create table X (id int primary key, name char(10));

create table Y (bid int primary key, aid references A(id) on delete cascade);

X contains one row (111, 'Mike')

Y contains two rows (1000, 111), (2000, 111)**

I if removed row (2000,111) in table Y what would happen?

Would that row just be deleted or would it even allow me to delete anything because of the reference to the parent table?

Thanks

Answer 1

It would be deleted and nothing else would happen. Cascading deletes only go from the referenced table to the referencing table. So a delete on table X will cascade a delete down to table y, while a delete on table y has no impact on table x.

Answer 2

什么都不会发生，只有当你从表X中删除一行时，表Y中引用它的行才会被删除。

Answer 3

ON Delete cascade option wont effect anything if you perform any delete on the child table. This option is used to specify, when you delete a row in the parent table, the database server also deletes any rows associated with that row (foreign keys) in a child table. The principal advantage to the cascading-deletes feature is that it allows you to reduce the quantity of SQL statements you need to perform delete actions.