MySQL-连接多个表

Question

I have tables shown below. I am using this for my android quiz application. I want to fetch the students who cant take the quiz.

Table quiz

+-------------------------------------------------------------------+
| id | title  |   start    |    end     | class_name | user_faculty |
+-------------------------------------------------------------------+
| 1  | Quiz 1 | 2016-08-01 | 2016-08-03 | T4         | faculty_1    |
+-------------------------------------------------------------------+
| 2  | Quiz 2 | 2016-08-01 | 2016-08-03 | T4         | faculty_1    |
+-------------------------------------------------------------------+
| 3  | Quiz 3 | 2016-08-03 | 2016-08-04 | T4         | faculty_1    |
+-------------------------------------------------------------------+

Table studentclass

+-----------------------------------------------+
| id | class_name | user_faculty | user_student |
+-----------------------------------------------+
| 1  | T4         | faculty_1    | student_1    |
+-----------------------------------------------+
| 2  | T4         | faculty_1    | student_2    |
+-----------------------------------------------+

My initial query here is:

SELECT Q.id, Q.title, Q.start, Q.end
FROM quiz Q
INNER JOIN studentclass SC
ON Q.class_name = SC.class_name
AND Q.start_on <= now() 
AND Q.end_on >= now()
AND SC.user_student = 'student_1'

consider that the date today is 2016-08-02

I am able to fetch the first and second row of the quiz table

+---------------------------------------+
| id | title  |   start    |    end     |
+---------------------------------------+
| 1  | Quiz 1 | 2016-08-01 | 2016-08-03 |
+---------------------------------------+
| 2  | Quiz 2 | 2016-08-01 | 2016-08-03 |
+---------------------------------------+

Now I also want to fetch the score table together with this previous query. A student 'student_1' finished taking the quiz.

Table score

+--------------------------------------------------------+
| id | score | grade | user_student | quiz_id | status   |
+--------------------------------------------------------+
| 1  | 10    | 100   | student_1    | 1       | Finished |
+--------------------------------------------------------+

My new query

SELECT Q.id, Q.title, Q.start, Q.end, S.user_student, S.status
FROM quiz Q
INNER JOIN studentclass SC
ON Q.class_name = SC.class_name
AND Q.start_on <= now() 
AND Q.end_on >= now()
AND SC.user_student = 'student_1'
LEFT JOIN score S
ON Q.id = S.quiz_id
WHERE S.quiz_id IS NULL

the result is

+---------------------------------------------------------------+
| id | title  |   start    |    end     | user_student | status |
+---------------------------------------------------------------+
| 2  | Quiz 2 | 2016-08-01 | 2016-08-03 | NULL         | NULL   |
+---------------------------------------------------------------+

the result is going fine but when I change the student_1 to student_2 in my 2nd query I am still getting the same result. The student_2 should see both of the quiz rows because he haven't taken the quiz yet.

SELECT Q.id, Q.title, Q.start, Q.end, S.user_student, S.status
FROM quiz Q
INNER JOIN studentclass SC
ON Q.class_name = SC.class_name
AND Q.start_on <= now() 
AND Q.end_on >= now()
AND SC.user_student = 'student_2' //this is where I changed the student_1 to student_2
LEFT JOIN score S
ON Q.id = S.quiz_id
WHERE S.quiz_id IS NULL

Thank you for your answers

Answer 1

It seems that the problem is that you don't check which student has taken the quiz when joining to the SCORE table. Thus a quiz is excluded from the result if it is completed by any student.

Try changing the query like this:

SELECT Q.id, Q.title, Q.start, Q.end, S.user_student, S.status
FROM quiz Q
INNER JOIN studentclass SC
ON Q.class_name = SC.class_name
AND Q.start_on <= now() 
AND Q.end_on >= now()
LEFT JOIN score S
ON Q.id = S.quiz_id
AND SC.user_student = S.user_student   -- Do not consider other students 
WHERE S.quiz_id IS NULL
AND SC.user_student = 'student_2'      -- moved from join clause for clarity