Mysql group_concat limit rows in grouping - mysql

Question

The next example is my database.
tb_port
id port
1 80
2 22
3 53
4 3128
5 443
tb_dest
id dest
1 network
2 local
tb_rule
id id_port id_dest
1 1 1
2 2 1
3 3 1
4 4 1
5 5 1
Select:
select dest,group_concat(port) from tb_port a, tb_dest b, tb_rule c where a.id=c.id_port and b.id=c.id_dest group by dest
Result:
network 80,22,53,3128,443
but is not the result I'm looking for, the result would be this.
Select ex:
select dest,group_concat(port limit 2) from tb_port a, tb_dest b, tb_rule c where a.id=c.id_port and b.id=c.id_dest group by dest
result I would like
network 80,22
network 53,3128
network 443
how to achieve this result only with SQL?
Sqlfiddle: http://sqlfiddle.com/#!2/d11807

Answer 1

MySQL doesn't make this kind of query easy, but one (admittedly not very pretty) solution is to use a variable to give each row a sequence number per dest and just group by the row number integer divided by 2 to get two numbers in each group;
SELECT dest, GROUP_CONCAT(port ORDER BY rank) ports
FROM (
SELECT dest, port, (
CASE dest WHEN #curDest
THEN #curRow := #curRow + 1
ELSE #curRow := 1 AND #curDest := dest END) rank
FROM tb_port a
JOIN tb_rule c ON a.id = c.id_port
JOIN tb_dest b ON b.id = c.id_dest,
(SELECT #curRow := 0, #curDest := '') r
ORDER BY dest
) z
GROUP BY FLOOR(rank/2),dest
ORDER BY dest, MIN(rank)
An SQLfiddle to test with.

Answer 2

Here is a stored proc,you just put in the delimiter when you call it
DELIMITER $$
DROP PROCEDURE IF EXISTS explode_table $$
CREATE PROCEDURE explode_table(bound VARCHAR(255))
BEGIN
DECLARE id TEXT;
DECLARE value TEXT;
DECLARE occurance INT DEFAULT 0;
DECLARE i INT DEFAULT 0;
DECLARE splitted_value TEXT;
DECLARE done INT DEFAULT 0;
DECLARE cur1 CURSOR FOR
select dest,group_concat(port) from tb_port a, tb_dest b, tb_rule c
where a.id=c.id_port and b.id=c.id_dest and dest != '' group by dest;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
DROP TEMPORARY TABLE IF EXISTS table2;
CREATE TEMPORARY TABLE table2(
`id` VARCHAR(255),
`value` VARCHAR(255) NOT NULL
) ENGINE=Memory;
OPEN cur1;
read_loop: LOOP
FETCH cur1 INTO id, value;
IF done THEN
LEAVE read_loop;
END IF;
SET occurance = (SELECT LENGTH(CONCAT(value,bound))
- LENGTH(REPLACE(CONCAT(value,bound), bound, ''))
+1);
SET i=2;
WHILE i <= occurance DO
SET splitted_value =
SUBSTRING_INDEX(SUBSTRING_INDEX(CONCAT(value,bound),bound,i),bound,-2) ;
INSERT INTO table2 VALUES (id, splitted_value);
SET i = i + 2;
END WHILE;
END LOOP;
SELECT * FROM table2;
CLOSE cur1;
END; $$
CALL explode_table(',')