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Fredric von Stange
3,867 PointsAdvanced Solution to Multi-Subject Teachers
I've come up with a detailed solution to the Multi-Subject Teachers query.
SELECT TEACHER_ID, FIRST_NAME, LAST_NAME, SUBJECT_ID, SUBJECTS.NAME FROM CLASSES
INNER JOIN SUBJECTS ON SUBJECTS.ID = CLASSES.SUBJECT_ID
--This derived table query grabs the Teacher IDs of any teacher who teachers more than one class, that way the SELECT query above only looks at those specific teachers who teach more than one class.
INNER JOIN (
SELECT DISTINCT TEACHERS.ID, FIRST_NAME,LAST_NAME, COUNT(DISTINCT SUBJECT_ID) AS CNT FROM TEACHERS
INNER JOIN CLASSES ON CLASSES.TEACHER_ID = TEACHERS.ID
INNER JOIN SUBJECTS ON SUBJECTS.ID = CLASSES.SUBJECT_ID
GROUP BY LAST_NAME HAVING CNT > 1
) AS MULTSUB ON MULTSUB.ID = CLASSES.TEACHER_ID
GROUP BY SUBJECT_ID;
1 Answer
Steven Parker
231,269 PointsI'm guessing the "detail" you're after is the names of the subjects taught, and that code does the job but at the cost of a separate line for each subject. There's a way to do it on one line per teacher (and without the derived table) using a function called "GROUP_CONCAT" that wasn't covered in the course:
SELECT TEACHERS.*, GROUP_CONCAT(DISTINCT SUBJECTS.NAME) AS SUBJECTS FROM TEACHERS
INNER JOIN CLASSES ON CLASSES.TEACHER_ID = TEACHERS.ID
INNER JOIN SUBJECTS ON SUBJECTS.ID = CLASSES.SUBJECT_ID
GROUP BY TEACHER_ID HAVING COUNT(DISTINCT SUBJECT_ID) > 1