CSV formatted GROUP_CONCAT in MySQL - mysql

Question

Let's say I have a Table A that I want to transform into Table B.
The values in Table B should always be a CSV formated text with the same number of fields.
First, I need to know what is the largest number of values that a given category handles (in this case, 3 values in category 1, 2 and 4);
Secondly I also need to use that variable to "add" empty fields(",") to the end of the GROUP_CONCAT when a category has "missing" values.
I need this to have a "consistent" CSV in each cell. The application I'm using to process this data doesn't interpret well CSVs with different column number by row...
Table A
+----+----------+-------+
| id | category | value |
+----+----------+-------+
| 1 | 1 | a |
| 2 | 1 | b |
| 3 | 1 | c |
| 4 | 2 | d |
| 5 | 2 | e |
| 6 | 2 | f |
| 7 | 3 | g |
| 8 | 3 | h |
| 9 | 4 | i |
| 10 | 4 | j |
| 11 | 4 | k |
| 12 | 5 | l |
+----+----------+-------+
Table B
+--------------+---------------------+
| id(category) | value(group_concat) |
+--------------+---------------------+
| 1 | a,b,c |
| 2 | d,e,f |
| 3 | g,h, |
| 4 | i,j,k |
| 5 | l,, |
+--------------+---------------------+
EDITED (SQLFiddle):
http://sqlfiddle.com/#!2/825f8

Answer 1

first, to get the largest number of values that a given category handles:
select count(category) from tableA group by category order by count(category) desc limit 1;
second, to add empty fields(",") to the end of the GROUP_CONCAT when a category has "missing" values.
i created a function called unify_length to help do this.
this is the function:
delimiter $$
CREATE FUNCTION `unify_length`(csv_list CHAR(255), length INT) RETURNS char(255)
DETERMINISTIC
BEGIN
WHILE ((SELECT LENGTH(csv_list) - LENGTH(REPLACE(csv_list, ',', ''))) < length-1) DO /* count the number of occurrances in a string*/
SET csv_list = CONCAT(csv_list, ',');
END WHILE;
RETURN csv_list;
END$$
and this is the function call:
select category, unify_length(GROUP_CONCAT(value), length) from tablea group by category;
where length is what was returned from the first query.