MySQL的加入通過

Question

使用組這些都是我的表

mysql> select * from professor; 
    +-------+--------+--------+--------+------+ 
    | empid | name | status | salary | age | 
    +-------+--------+--------+--------+------+ 
    |  1 | Arun |  1 | 2000 | 23 | 
    |  2 | Benoy |  0 | 3000 | 25 | 
    |  3 | Chacko |  1 | 1000 | 36 | 
    |  4 | Divin |  0 | 5000 | 32 | 
    |  5 | Edwin |  1 | 2500 | 55 | 
    +-------+--------+--------+--------+------+ 
    5 rows in set (0.00 sec) 

mysql> select * from works; 
+----------+-------+---------+ 
| courseid | empid | classid | 
+----------+-------+---------+ 
|  1 |  1 |  10 | 
|  2 |  2 |  9 | 
|  3 |  3 |  8 | 
|  4 |  4 |  10 | 
|  5 |  5 |  9 | 
|  6 |  1 |  9 | 
|  2 |  3 |  10 | 
|  2 |  1 |  7 | 
|  4 |  2 |  6 | 
|  7 |  5 |  6 | 
|  3 |  5 |  2 | 
|  2 |  4 |  6 | 
|  2 |  5 |  2 | 
+----------+-------+---------+ 
15 rows in set (0.00 sec) 

mysql> select * from course; 
+----------+------------+--------+ 
| courseid | coursename | points | 
+----------+------------+--------+ 
|  1 | Maths  |  4 | 
|  2 | Science |  4 | 
|  3 | English |  85 | 
|  4 | Social  |  4 | 
|  5 | Malayalam |  99 | 
|  6 | Arts  |  40 | 
|  7 | Biology | 100 | 
+----------+------------+--------+ 
7 rows in set (0.00 sec) 

的問題是：

Return the names of full professors who have taught at least two courses in one Class 

我的查詢是：

select professor.name from professor 
inner join works 
on professor.empid=works.empid 
group by works.empid 
having count(distinct works.courseid)>=2 

我現在得到的輸出繼電器是：

Arun 
Benoy 
Chacko 
Divin 
Edwin 

我被認爲是'Edwin'的輸出，因爲他是唯一一個在同一班上教過兩門科目的人。請幫助

Answer 1

你必須考慮你的小組是錯改變這個

group by classid,works.empid 

一樣，你會在數CLASSID courseid。