示例：

SELECT
 identity,
 GROUP_CONCAT( `key`, ':', `value` ) AS identity_data 
FROM
 oc_product_gmc_config 
WHERE
 product_id = '28' 
GROUP BY
 `identity`

输出：

identity	identity_data
custom_label	custom_label_0:0,custom_label_1:1,custom_label_2:2,custom_label_3:3,custom_label_4:4

这样就可以获得分组内每个记录的数据。