Question

I have two mysql tables, courses and student_courses, I need to get unique course_names from the course table. But I get a erro when trying to execute my query.

Heres my query

        SELECT
            sc.c_id,
            DISTINCT c.course_name
        FROM
            Courses AS c
        LEFT JOIN Student_Courses AS sc ON c.c_id = sc.c_id
        WHERE
            sc.s_id = 4

This is the error I get

CDbCommand failed to execute the SQL statement: SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DISTINCT c.course_name

FROM
Courses AS c
LEFT JOIN Student_Courses AS ‘ at line 3

Answer 1

DISTINCT is a keyword to be used immediately after SELECT (Have a look at the Mysql SELECT syntax). You cannot select columns distinctively, you rather do a SELECT DISTINCT query. What you possibly want is a GROUP BY. But then you need an aggregate function (to tell mysql what to do with the other columns – the ones you are not GROUPing BY.) Example using GROUP_CONCAT as aggregate function (Which just concatenates all columns in each group using a comma):

SELECT c.course_name, GROUP_CONCAT(sc.c_id),
FROM Courses AS c
LEFT JOIN Student_Courses AS sc ON c.c_id = sc.c_id
WHERE sc.s_id = 4
GROUP BY course_name