连接字段具有多个匹配项时的MYSQL聚合条目

Question

我试图为可能会与给定老师一起参加多个课程的学生计算答案总数，以及对给定问题的正确答案总数。 我希望将结果按测试和老师分组，并删除重复的学生。

我要摆脱的是：

test1-teacher1-6-4
test1-teacher2-3-2

但是，由于student3，student4和student5与Teacher1一起进入不同的班级（见下文），导致他们被计数两次，所以我得到...

test1-teacher1-6-6
test1-teacher2-3-2

这是我正在使用的所有数据。

关于如何删除重复的学生有什么想法？ 谢谢。

create database test_db;

use test_db;

drop table if exists test;
create table test(test_name varchar (50),student_name varchar(50),result varchar(50));


drop table if exists roster;
create table roster(teacher_name varchar(50),student_name varchar(50), class_name varchar(50));

insert into test values ('test1','student1','c');
insert into test values ('test1','student2','x');
insert into test values ('test1','student3','x');
insert into test values ('test1','student4','c');
insert into test values ('test1','student5','c');
insert into test values ('test1','student6','c');
insert into test values ('test1','student7','c');
insert into test values ('test1','student8','x');

insert into roster values ('teacher1', 'student1', 'class1'); 
insert into roster values ('teacher1', 'student2', 'class1'); 
insert into roster values ('teacher1', 'student3', 'class1');
insert into roster values ('teacher1', 'student4', 'class1');
insert into roster values ('teacher1', 'student5', 'class1');
insert into roster values ('teacher1', 'student6', 'class1');

insert into roster values ('teacher1', 'student3', 'class2');
insert into roster values ('teacher1', 'student4', 'class2');
insert into roster values ('teacher1', 'student5', 'class2');

insert into roster values ('teacher2', 'student6', 'class3');
insert into roster values ('teacher2', 'student7', 'class3');
insert into roster values ('teacher2', 'student8', 'class3');



use test_db;
select 
    test_name,
    r.teacher_name,
    count(distinct r.student_name) as numTested,
    sum(case
        when result = 'c' then 1
        else null
    end) as Q1correct
from
    test t
        join
    roster r ON t.student_name = r.student_name
group by t.test_name , r.teacher_name;

Answer 1

使用子查询从roster删除重复项。

select 
    test_name,
    r.teacher_name,
    count(distinct r.student_name) as numTested,
    sum(case
        when result = 'c' then 1
        else null
    end) as Q1correct
from
    test t
        join
    (select distinct teacher_name, student_name
     from roster) r ON t.student_name = r.student_name
group by t.test_name , r.teacher_name;

SQLFIDDLE