Question

I’m a high school teacher new to php|mySQL and I am trying to develop a query to display test scores from my students. The challenge is to show the scores on a given test and to show NULL if a score has not been posted. I am trying to make two steps into a single query:

1. Create a list of students in a particular course based on submitted test results.
2. Display each student’s score on that test, or, if a score doesn’t exist because they haven’t taken a test, display their name and NULL next to it.

The ‘tests’ table with records for each assessment contains these columns:

test_id
assessment_id
username
score
maxScore
stopTime

The ‘users’ table with records for each student contains these columns:

user_id
schoolId
username
first_name
last_name
section

Desired Result:

username | last_name | first_name | schoolId | assessment_id | score | maxScore
------------------------------------------------------------------------------------
jadams1  | Adams     | Joe        | 111111   | 23            | 9     | 12
sbenson1 | Benson    | Sally      | 222222   | 23            | 10    | 12
csmith2  | Smith     | Charlie    | 555555   | NULL          | NULL  | NULL (hasn't taken quiz yet)
dthomp1  | Thompson  | David      | 666666   | 23            | 9     | 12

So far I have tried a number of things. The first was a basic LEFT JOIN. This generates one row for each student, but since I specify a value for “tests.assessment_id”, it does not display rows for students who haven’t taken the quiz yet. I want it to display the quiz results if they exist, and a row for the student containing NULL score values if they haven’t taken the quiz.

SELECT
  users.last_name,
  users.first_name,
  tests.score,
  tests.maxScore
FROM users
  LEFT JOIN tests
    ON tests.username = users.username
WHERE tests.assessment_id = '23'
    AND users.section = 'S432-01'

The above generates:

username | last_name | first_name | schoolId | assessment_id | score | maxScore
------------------------------------------------------------------------------
jadams1  | Adams     | Joe        | 111111   | 23            | 9     | 12
sbenson1 | Benson    | Sally      | 222222   | 23            | 10    | 12
dthomp1  | Thompson  | David      | 666666   | 23            | 9     | 12

The following is a more recent iteration. It generates a NULL row for practice quizzes (with no assessment_id assigned) as well as a row for the real thing. In the final report, it is important to create only one row for each student, but not to miss any students:

SELECT DISTINCT
  users.username,
  users.last_name,
  users.first_name,
  users.schoolId,
  if(tests.assessment_id ='23','23','') AS assessment,
  if(tests.assessment_id ='23',tests.score,'') AS score,
  if(tests.assessment_id ='23',tests.maxScore,'') AS maxScore
FROM `users`
  LEFT JOIN tests
    ON tests.username = users.username
WHERE users.section = 'S432-01'
    AND (tests.assessment_id = '23'
      OR tests.assessment_id IS NULL)
ORDER BY users.last_name,users.first_name,tests.assessment_id

The above generates:

username | last_name | first_name | schoolId  | assessment_id | score | maxScore
------------------------------------------------------------------------------------
jadams1  | Adams     | Joe        | 111111    | NULL          | NULL  | NULL
jadams1  | Adams     | Joe        | 111111    | 23            | 9     | 12
sbenson1 | Benson    | Sally      | 222222    | NULL          | NULL  | NULL
sbenson1 | Benson    | Sally      | 222222    | 23            | 10    | 12
csmith2  | Smith     | Charlie    | 555555    | NULL          | NULL. | NULL (hasn't taken quiz yet)
dthomp1  | Thompson  | David      | 666666    | NULL          | NULL. | NULL
dthomp1  | Thompson  | David      | 666666    | 23            | 9     | 12

Any ideas how to make this work in a single query? I think I’m having performance problems, so I would like to keep things as simple as possible.

Thanks for your help!
-Chris

Answer 1

This will work,

SELECT  users.last_name,
        users.first_name,
        tests.score,
        tests.maxScore
FROM    users
        LEFT JOIN tests
              ON tests.username = users.username AND 
                 tests.assessment_id = '23'
WHERE   users.section = 'S432-01'

The reason why you are not getting all records from users table is because you have filtered assessment_id which is equal to 23 which drops all the null values (students that have not taken the assessment has null value on this column). In order to get all values, move the tests.assessment_id='23' on the ON clause.