Question

I’m trying to build a SQL Union Query which would return two columns (one per select) with the result from the same field, sorted with the WHERE clause.

Here is what I trying :

SELECT scg.grade AS course_1
FROM cp_StudentCourseGrade AS scg
JOIN cp_CourseSecSem AS css ON
css.courseSecSem_id = scg.courseSecSem_id
JOIN cp_Course AS c1 ON
c1.course_id = css.course_id
UNION
SELECT scg.grade AS course_2
FROM cp_StudentCourseGrade AS scg
JOIN cp_CourseSecSem AS css ON
css.courseSecSem_id = scg.courseSecSem_id
JOIN cp_Course AS c2 ON
c2.course_id = css.course_id**
WHERE c1.course_id = c2.course_id AND c1.course_id=1 AND c2.course_id=2;

and this should return me something like that :

    course_1 | course_2
    -------------------
    4.0      | 6.0
    4.5      | 6.0
    6.0      | 5.5
    2.5      | 5.0
    1.0      | 4.0
    5.5      | 4.5

But I am having an error that says :

unknown column c1.course_id

Does anyone have an idea how I should do it?

Answer 1

This is what you need:

SELECT scg1.grade AS course_1, scg2.grade AS course_2
FROM cp_StudentCourseGrade AS scg1
JOIN cp_CourseSecSem AS css1 ON
css1.courseSecSem_id = scg1.courseSecSem_id
JOIN cp_Course AS c1 ON
c1.course_id = css1.course_id AND c1.course_id=1 
JOIN cp_StudentCourseGrade AS scg2
JOIN cp_CourseSecSem AS css2 ON
css2.courseSecSem_id = scg2.courseSecSem_id
JOIN cp_Course AS c2 ON
c2.course_id = css2.course_id AND c2.course_id=2;