How do I update query two tables and update in one table? - mysql

Question

I have two tables, candidates and candidate_subjects, for storing candidate details and candidate scores respectively. I want a query to update candidate remark to 'FAIL' if the candidate pass less than 6 subjects. To pass a subject, the candidate sum of ca_score and exam_score for a subject must be greater than 40.
Below is the query I have written but it is not giving the result expected:
UPDATE candidates SET candidates.remark='FAIL' WHERE (select
count(candidate_subjects.id) AS total_pass from candidates,
candidate_subjects where candidates.id=candidate_subjects.candidate_id
and (candidate_subjects.ca_score + candidate_subjects.exam_score) >= 40) < 6
The tables:
CREATE TABLE candidate_subjects (
id INT(10) PRIMARY KEY NOT NULL AUTO_INCREMENT,
candidate_id INT(11),
exam_type_id INT(10),
subject_id INT(10),
ca_score INT(11),
exam_score INT(6),
score_grade VARCHAR(10),
date_created VARCHAR(10),
date_modified TIMESTAMP
);
INSERT INTO `candidate_subjects` (`id`, `candidate_id`, `exam_type_id`,
`subject_id`, `ca_score`, `exam_score`, `score_grade`, `date_created`,
`date_modified`) VALUES
(1, 2, 1, 32, 22, 61, NULL, '2017-02-01', '2017-08-28 13:10:33'),
(2, 2, 1, 5, 21, 38, NULL, '2017-02-01', '2017-08-28 13:10:33'),
(3, 2, 1, 14, 21, 51, NULL, '2017-02-01', '2017-08-28 13:10:33'),
(4, 2, 1, 1, 19, 34, NULL, '2017-02-01', '2017-08-28 13:10:33'),
(5, 2, 1, 2, 23, 39, NULL, '2017-02-01', '2017-08-28 13:10:33'),
(6, 2, 1, 38, 20, 32, NULL, '2017-02-01', '2017-08-28 13:10:33'),
(7, 2, 1, 53, 24, 47, NULL, '2017-02-01', '2017-08-28 13:10:33'),
(8, 4, 1, 32, 19, 61, NULL, '2017-02-01', '2017-08-28 13:11:27'),
(9, 4, 1, 5, 22, 41, NULL, '2017-02-01', '2017-08-28 13:11:27'),
(10, 4, 1, 14, 20, 46, NULL, '2017-02-01', '2017-08-28 13:11:27'),
(11, 4, 1, 1, 23, 37, NULL, '2017-02-01', '2017-08-28 13:11:27'),
(12, 4, 1, 2, 21, 36, NULL, '2017-02-01', '2017-08-28 13:11:27'),
(13, 4, 1, 38, 22, 34, NULL, '2017-02-01', '2017-08-28 13:11:27'),
(14, 4, 1, 53, 24, 52, NULL, '2017-02-01', '2017-08-28 13:11:27'),
(15, 5, 1, 32, 20, 62, NULL, '2017-02-01', '2017-08-28 13:11:44'),
(16, 5, 1, 5, 22, 38, NULL, '2017-02-01', '2017-08-28 13:11:44');
CREATE TABLE candidates (
id INT(11) PRIMARY KEY NOT NULL AUTO_INCREMENT,
exam_no VARCHAR(15),
surname VARCHAR(50),
other_names VARCHAR(100),
school_id INT(11),
registration_completed INT(11),
exam_scores_completed INT(5),
remark VARCHAR(10)
);
INSERT INTO candidates (id, exam_no, surname, other_names, school_id,
registration_completed, exam_scores_completed, remark) VALUES
(1, '1171052001', 'ABADO', 'MASENENGEN', 1052, 1, '1', ''),
(2, '1170938001', 'AGBA', 'NGUHER', 938, 1, '1', ''),
(3, '1170071001', 'ABEE', 'SESUUR', 71, 1, '1', ''),
(4, '1170938002', 'AHEN', 'REBECCA DOOSUUN', 938, 1, '1', '');

Answer 1

To update your candidates according to your criteria and their records exists in candidate_subjects you can use below query.
UPDATE candidates c
JOIN (select
count(id) AS total_pass ,candidate_id
from candidate_subjects
where(ca_score + exam_score) >= 40
group by candidate_id
) a on c.id = a.candidate_id
SET c.remark='FAIL'
WHERE total_pass < 6
DEMO

Answer 2

Can you try something like:
UPDATE candidates SET candidates.remark='FAIL' WHERE candidate_id IN (SELECT candidate_id FROM candidate_subject WHERE candidatescorecondition>40 HAVING COUNT(*) > 6);
Please run in a dev environment first!

Answer 3

If candidates can be marked as FAIL without doing all the exams you can do something like this:
update candidates set remark='FAIL'
where id in (
select candidate_id from candidate_subjects
where ca_score+exam_score > 40 -- passed subject
group by candidate_id having count(*)<6
)
but if a candidate must have data for at least six subjects in order to be evaluated you need to add an extra condition (so a candidate is not marked as failed if he/she is in the middle of the process):
update candidates set remark='FAIL'
where id in (
select candidate_id from candidate_subjects
where ca_score+exam_score > 40 -- passed subject
group by candidate_id having count(*)<6
)
and id in (
select candidate_id from candidate_subjects
group by candidate_id having count(*)>=6 -- the candidate has data for at leas 6 subjects
)