用过Mysql的都知道她有一个很好的实现行转列功能的函数group_concat函数，非常方便
SELECT
*
FROM
group_test;
SELECT
id,
GROUP_CONCAT(sub_id)
FROM
`group_test`
GROUP BY
id;
现在的需求是有上面图二类似的结果集，需要把列二拆分 转换成行记录
我们知道如果是单条记录通过SUBSTRING_INDEX容易实现
select id,SUBSTRING_INDEX(sub_id,',',1) from group_test where id=3
UNION
select id,SUBSTRING_INDEX(SUBSTRING_INDEX(sub_id,',',2),',',-1) from group_test where id=3
UNION
select id,SUBSTRING_INDEX(SUBSTRING_INDEX(sub_id,',',3),',',-1) from group_test where id=3
但是如果是N条呢？同样也是可以使用SUBSTRING_INDEX来实现，只不过需要一个配置表，通过CROSS JOIN交叉连接实现，先看下CROSS JOIN
SELECT
*
FROM
(SELECT 1 UNION SELECT 2) t1
CROSS JOIN (SELECT 3 UNION SELECT 4) t2
下面就通过CROSS JOIN和SUBSTRING_INDEX实现我们的需求，首先构建一个配置表
CREATE TABLE digits (digit INT(1));
INSERT INTO digits
VALUES
(0),
(1),
(2),
(3),
(4),
(5),
(6),
(7),
(8),
(9);
CREATE TABLE sequence (seq INT(3));
INSERT INTO sequence (
SELECT
D1.digit + D2.digit * 10
FROM
digits D1
CROSS JOIN digits D2
);
然后
SELECT
id,
SUBSTRING_INDEX(
SUBSTRING_INDEX(sub_id, ',', seq),
',' ,- 1
) sub_id,
seq
FROM
sequence
CROSS JOIN group_test
WHERE
seq BETWEEN 1
AND (
SELECT
1 + LENGTH(sub_id) - LENGTH(REPLACE(sub_id, ',', ''))
)
ORDER BY
id,
sub_id;
原贴: http://blog.chinaunix.net/uid-411974-id-3990697.html