mysql joins using group by

Question

These are my tables

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)

Question is :

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

My query is :

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

The ouput I get now is :

Arun
Benoy
Chacko
Divin
Edwin

I am supposed to get the ouput as 'Edwin' as he is the only person who has taught 2 subjects in the same class. Pls help

Answer 1

you must consider your group by is wrong change it to this

   group by classid,works.empid

like that you will count courseid in classid .