mysql select multitable - join

Question

说我有下表

user_table 
id    username
1     abc
2     def
3     ghij

courses_table 
id    title
1     csc
2     math
3     syn

user_courses 
user_id    course_id
2           1
1           3
2           3

我想选择参加课程1和3的用户名,而不是至少1或3,我的意思是1和3

我尝试了以下mysql查询

SELECT DISTINCT  u.* FROM  user_table as u  LEFT JOIN user_courses as uc ON uc.user_id = u.id  WHERE uc.course_id = 1 AND uc.course_id=3;
SELECT DISTINCT  u.* FROM  user_table as u  LEFT JOIN user_courses as uc ON uc.user_id = u.id  WHERE uc.course_id IN (1,3);
SELECT DISTINCT  u.* FROM  user_table as u  LEFT JOIN user_courses as uc ON uc.user_id = u.id  WHERE uc.course_id IN (1,3) AND uc.user_id = u.id ;

执行的第一个和第三个查询未显示任何结果,第二个查询显示至少具有course_id 1或3的所有用户

如果你想知道为什么我使用LEFT JOIN,这是因为我需要连接表的结果,上面的代码行只是一个例子,我用来从大约9个表中使用LEFT连接获取数据.

有什么帮助吗？谢谢

SELECT DISTINCT  u.* FROM  user_table as u  LEFT JOIN user_courses as uc ON uc.user_id = u.id  WHERE uc.course_id IN( 1,3) AND uc.user_id = 2 ";

这告诉我我想要的结果,它的输出"def",但我不能使用user_id作为静态值(在这个例子中为数字2)

Answer 1

这个问题被称为 Relational Division

SELECT  a.id, a.username
FROM    user_table a
        INNER JOIN user_courses b
            ON a.id = b.user_ID
WHERE   b.course_ID IN (1,3)
GROUP   BY a.id, a.username
HAVING  COUNT(*) = 2

• 关系部门的SQL

如果course_ID考虑到用户重新参加课程,并非每个用户都是唯一的,那么DISTINCT需要一个关键字来选择独特的课程,

SELECT  a.id, a.username
FROM    user_table a
        INNER JOIN user_courses b
            ON a.id = b.user_ID
WHERE   b.course_ID IN (1,3)
GROUP   BY a.id, a.username
HAVING  COUNT(DISTINCT b.course_ID) = 2

• SQLFiddle演示

OUTPUT

?????????????????
? ID ? USERNAME ?
?????????????????
?  2 ? def      ?
?????????????????