Question

consider three entities as student, course, subject
Below are the associations –

student has_many courses,
student has_many subjects.

Now i want to fetch student records with subject names and course names using mysql group_concat, left join on courses, left join on subjects and group_by student_id.

Problem is that group_concat('subjects.name') as subject_names gives me duplicate entries of subjects but group_concat('students.name') as student_names gives unique names.

Why ??

Answer 1

The 2 left joins are multiplying rows via Cartesian product of the child rows per student

Example

• Student 1 has 3 courses and 2 subjects
• Generates 6 rows for Student 1
• Gives one course value per subject = each course repeated twice
• Gives one subject value per course = each subject repeated thrice

To fix:

Option 1: Use GROUP_CONCAT(DISTINCT ...) as per MySQL docs

In MySQL, you can get the concatenated values of expression combinations. To eliminate duplicate values, use the DISTINCT clause.

Option 2: Use a UNION ALL + derived table

SELECT
    Student, MAX(CourseConcat), MAX(SubjectConcat)
FROM
    (
    -- 2 separate SELECTs here
    .. student LEFT JOIN course ...
    UNION ALL
    .. student LEFT JOIN subjects...
    ) T
GROUP BY
   Student

The 2nd option may be better albeit more complex because you have less intermediate rows to process with DISTINCT