create table answers
(
user_id int not null,
question_id int not null,
content text not null,
answer_date datetime not null,
primary key(answer_date),
foreign key(user_id,question_id) references forum(user_id,question_id)
)
(
user_id int not null,
question_id int not null,
content text not null,
answer_date datetime not null,
primary key(answer_date),
foreign key(user_id,question_id) references forum(user_id,question_id)
)
foreign key(user_id) references ***users***(user_id),
foreign key(question_id) references ***forum***(question_id)
外键对应的表有两个:users和forum,不是users。
foreign key(question_id) references forum(question_id)是不正确的。