mysql - Max occurences of a given value in a table - php

Question

I have a table like this
+-------+-------+-------+-------+
| id | cid | grade |g_point|
+-------+-------+-------+-------+
| 1 | 10 | A+ | 1 |
| 2 | 10 | A+ | 1 |
| 3 | 10 | B | 3 |
| 4 | 11 | A | 2 |
| 5 | 11 | A+ | 1 |
| 6 | 12 | B | 3 |
the column g_point is the values associated to each grade. forexample A+ grade considers highest so I assign the value of A+ is one(highest starts from 1 to 10) and so on. These g_point values are constant. Now what I want to do is I want to show the maximum grade against each course and also if somehow there are only two entries of different grades I want to compare it with the g_point and choose whose value is lower because lower integer value means higher grade. the result should be like this and also sorted from top grade to lower.
+-------+-------+
| cid | grade |
+-------+-------+
| 10 | A+ |
| 11 | A+ |
| 12 | B |
I have tried this query
SELECT coursecodeID AS cid, (SELECT grade
FROM feedback
WHERE coursecodeID = cid
GROUP BY grade
ORDER BY COUNT(*) DESC LIMIT 0,1) AS g
FROM feedback
GROUP BY coursecodeID
but in this query I don't know how can I compare it with g_point value and also the courses is not showing in order(from highest grade to lowest).
NOTE: I want to choose the grade having the maximum number of occurrences per course id. For example here in this table course id 10 has 2 A+ grade so we'll consider A+ and if clash happens like one is A+ and the other is B+, then we'll have to compare it with the g_point

Answer 1

This works, but needs the 'g_point' to alse be returned.
SELECT cid,grade,MIN(g_point)
FROM grades
GROUP BY cid
This is more reliable, as it generates the Grade in the sub-query, and then appends it to the main table.
SELECT cid, (
SELECT grade
FROM grades g2
WHERE g2.cid = g1.cid
ORDER BY g_point
LIMIT 1
) AS grade
FROM grades g1
GROUP BY cid

Answer 2

You can use the following query:
SELECT DISTINCT m1.cid, m1.grade
FROM mytable AS m1
INNER JOIN (
SELECT cid, MIN(g_point) AS maxGrade
FROM mytable
GROUP BY cid ) m2
ON m1.cid = m2.cid AND m1.g_point = m2.maxGrade
The derived table contains the minimum g_point per cid. If you join it back to the original table, then you can get the maximum grade per cid.
Demo here
EDIT:
You can alternatively use a correlated sub-query:
SELECT cid, (SELECT grade
FROM mytable AS m2
WHERE m2.cid = m1.cid
ORDER BY g_point LIMIT 1) AS maxGrade
FROM mytable AS m1
GROUP BY cid
Demo here
EDIT2:
It looks like you want to get the grade having the maximum number of occurrences per cid. In case there are more than one grades sharing this maximum number, then fetch the grade with the lowest g_point.
You can do it using variables:
SELECT cid, grade
FROM (
SELECT cid, grade,
#row_number := IF (#cid <> cid,
IF (#cid := cid, 1, 1),
IF (#cid := cid, #row_number+1, #row_number+1)) AS rn
FROM (
SELECT cid, grade,
COUNT(*) AS cnt,
(SELECT g_point
FROM mytable AS m2
WHERE m1.grade = m2.grade
LIMIT 1) AS g_point
FROM mytable AS m1
GROUP BY cid, grade
) t
CROSS JOIN (SELECT #row_number:=-1, #cid:=-1) AS vars
ORDER BY cid, cnt DESC, g_point
) s
WHERE rn = 1
Demo here

Answer 3

Something to think about...
SELECT * FROM ints;
+---+
| i |
+---+
| 0 |
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
| 7 |
| 8 |
| 9 |
+---+
SELECT i
, CONCAT(CHAR((i/2)+64),IF(MOD(i,2)=1,'+',''))n
FROM ints
WHERE i > 0;
+---+------+
| i | n |
+---+------+
| 1 | A+ |
| 2 | A |
| 3 | B+ |
| 4 | B |
| 5 | C+ |
| 6 | C |
| 7 | D+ |
| 8 | D |
| 9 | E+ |
+---+------+