如果我删除用户 SQL，我可以删除用户的问题和答案吗

Question

我在 SQL 上有用户表问题表和答案表。 我想在删除级联问题上添加外键 user_ID 外键和回答 user_ID 外键，当我删除用户时，他的所有问题和答案也会被删除。但是 SQL 说：

在表“问题”上引入 FOREIGN KEY 约束“FK__questions__usID__1BC821DD”可能会导致循环或多个级联路径。指定 ON DELETE NO ACTION 或 ON UPDATE NO ACTION，或修改其他 FOREIGN KEY 约束。

我该怎么做。

Answer 1

您希望questions（resp：answers）表上的外键引用users 表，并在从users 中删除用户的所有问题（resp：答案）时删除它表。

这应该按照设计工作：

alter table answers add constraint answers_users 
    foreign key(user_id)
    references users(user_id)
    on delete cascade
;

alter table questions add constraint questions_users 
    foreign key(user_id)
    references users(user_id)
    on delete cascade
;

考虑 this demo：

-- create the tables
create table users (user_id int primary key);
create table questions(question_id int primary key, user_id int);
create table answers(answer_id int primary key, user_id int);

-- add the constraints
alter table answers add constraint answers_users 
    foreign key(user_id)
    references users(user_id)
    on delete cascade
;
alter table questions add constraint questions_users 
    foreign key(user_id)
    references users(user_id)
    on delete cascade
;

-- insert a few records - we have two users: 1 and 2
insert into users values (1), (2);
insert into questions values(1, 1), (2, 2);
insert into answers values(1, 1), (2, 2);

-- now delete user 1
delete from users where user_id = 1;

-- the corresponding record was removed from "questions"
select * from questions;

question_id | user_id
----------: | ------:
          2 |       2

-- ... and it was removed from "answers" as well
select * from answers;

answer_id | user_id
--------: | ------:
        2 |       2