Conditional group by in mysql - php

Question

I had a mysql query where i need to add some condition in Group by statement , if i use single field in Group by it works but i need two field include in the Group by, here is my query any one please help me to find out the issue
SELECT (CASE
WHEN CSR.skill_type = 1 THEN
(SELECT skills_value from cv_skills
WHERE skills_id = CSR.skill_id )
WHEN CSR.skill_type = 2 THEN
(SELECT ostype_name from cv_os_type
WHERE ostype_id = CSR.skill_id )
WHEN CSR.skill_type = 3 THEN
(SELECT dbtype_name from cv_db_type
WHERE dbtype_id = CSR.skill_id)
WHEN CSR.skill_type = 4 THEN
(SELECT title from candidate_competencies
WHERE id = CSR.skill_id)
WHEN CSR.skill_type = 0 THEN
IT.type_name
END) AS skill_name,
(CASE
WHEN IT.type_parent_id > 0 THEN
IT.type_parent_id
WHEN IT.type_parent_id = 0 THEN
CIS.interview_type
END) AS typeId,
(CASE
WHEN CSR.skill_type = 4 THEN
minimum_rating
END) AS minimum_rating,
AVG( CSR.rate ) AS skill_rating,
CSR.skill_type,CIS.created, CC.id
FROM `candidate_interview_skill_rate` `CSR`
LEFT JOIN `candidate_interview_process` `CIP` ON CSR.interview_process_id = CIP.id
LEFT JOIN `candidate_interview_schedule` `CIS` ON CIS.id = CIP.interview_schedules_id AND CIS.archive_date IS NULL
LEFT JOIN `candidate_interview` `CI` ON CI.id = CIS.interview_id AND CI.archived_date IS NULL
LEFT JOIN `interview_type` `IT` ON IT.id = CIS.interview_type
LEFT JOIN `candidate_competencies` `CC` ON CC.id = CSR.skill_id
WHERE CI.candidate_user_id = 39
GROUP BY (CASE
WHEN CSR.skill_type > 0 THEN
CSR.skill_id, CSR.skill_type
ELSE CIS.interview_type
END)
ORDER BY `CSR`.`skill_type`

Answer 1

You can concatenate CSR.skill_id, CSR.skill_type to a single column and use it in select statement. The group by needs to be like this
GROUP BY CASE
WHEN CSR.skill_type > 0 THEN
CONCAT(CSR.skill_id, '-', CSR.skill_type)
ELSE
CIS.interview_type
END;
Hope this works.

Answer 2

You can include multiple expressions in a GROUP BY clause, just separate the expressions with commas, just like in the SELECT list.
Very often, the expressions in the SELECT list are repeated in the GROUP BY clause.
It's often possible to use a CASE expression to support a more complex set of conditions.
It's hard to provide more concrete assistance with your query, absent sample data, and desired output.