Why i get mysql result column value dismatch - mysql

Question

msyql query
SELECT id,student_user_id,MIN(start_time) FROM appoint_course
WHERE student_user_id IN(
931,2034,2068,2111,2115,2173,2181,2285,2500,2505,2507,
2518,2594,2596,2600,2608,2637,2652,2654
)
AND course_type=3 and disabled=0 GROUP BY student_user_id;
result
[query result]
+-------+-----------------+-----------------+
| id | student_user_id | MIN(start_time) |
+-------+-----------------+-----------------+
| 8356 | 931 | 1500351000 |
| 9205 | 2034 | 1501733400 |
| 9246 | 2068 | 1501649100 |
| 9755 | 2111 | 1502943000 |
| 9585 | 2115 | 1502595300 |
| 10820 | 2173 | 1503545700 |
| 9594 | 2181 | 1502852400 |
| 10324 | 2285 | 1502852400 |
| 11204 | 2500 | 1504839600 |
| 11152 | 2507 | 1504064100 |
| 12480 | 2594 | 1505707800 |
| 11521 | 2608 | 1504494000 |
| 11818 | 2652 | 1504753200 |
+-------+-----------------+-----------------+
but right start time is:
id: 9594
start_time: 1503284400
9594 right start_time is 1503284400 not 1502852400.In fact 1502852400 is a record of 9597
I do not know why.

Answer 1

In any other database your query would return an error, because id is not in the group by. The correct query is:
SELECT student_user_id, MIN(start_time)
FROM appoint_course
WHERE student_user_id IN (931,2034,2068,2111,2115,2173,2181,2285,2500,2505,2507,2518,2594,2596,2600,2608,2637,2652,2654) AND
course_type = 3 and disabled = 0
GROUP BY student_user_id;
In your case, adding a simple MIN(id) to the SELECT might work, assuming that ids increase with the start time.
More generally, you appear to want:
SELECT ac.*
FROM appoint_course ac
WHERE ac.student_user_id IN (931,2034,2068,2111,2115,2173,2181,2285,2500,2505,2507,2518,2594,2596,2600,2608,2637,2652,2654) AND
ac.course_type = 3 AND ac.disabled = 0 AND
ac.start_time = (SELECT MIN(ac2.start_time)
FROM appoint_course ac2
WHERE ac2.student_user_id = ac.student_user_id AND
ac2.course_type = ac.course_type AND
ac2.disabled = ac.disabled
);
No GROUP BY is necessary.
I should add that there is a MySQL hack that often works:
SELECT student_user_id, MIN(start_time),
SUBSTRING_INDEX(GROUP_CONCAT(id ORDER BY start_time), ',', 1) as id_at_min_start_time
FROM appoint_course
WHERE student_user_id IN (931,2034,2068,2111,2115,2173,2181,2285,2500,2505,2507,2518,2594,2596,2600,2608,2637,2652,2654) AND
course_type = 3 and disabled = 0
GROUP BY student_user_id;
This uses string manipulations and the GROUP_CONCAT() can overflow internal buffer sizes.