试试这个：
样本数据：

create table users (uid int, name varchar(10), score int);
insert into users values
(1, 'john', 20),
(2, 'mary' , 40),
(3, 'david', 60),
(4, 'nancy', 80);
create table question_tb (qid int, question char(1));
insert into question_tb values
(1, 'a'),     
(2, 'b'),    
(3, 'c'),     
(4, 'd');   
create table question_user (quid int, user_id int, question_id int);
insert into question_user values
(1, 1, 1),
(2, 2, 1),
(3, 1, 2),
(4, 3, 3);

t-sql：

select uid, qid from users u
cross join question_tb q
where not exists(
    select 1 from question_user
    where u.uid = user_id and q.qid = question_id)
order by uid, qid

如果你想在desc中得到特定用户没有回答的问题，那么使用下面的查询

SELECT * FROM questions WHERE qid NOT IN (SELECT question_id FROM `question_user` WHERE uid = USER-ID) ORDER BY qid DESC;

下面的查询应该会给出用户尚未回答的问题。

SELECT *
FROM question_tb as q
LEFT JOIN question_user as qu on q.qid = qu.question_id AND qu.user_id = USER_ID_HERE
WHERE  qu.user_id IS NULL;