mysqlGROUP_CONCAT结果是blob类型的，怎么转换成varchar

select faudit_main_id,GROUP_CONCAT( CONCAT('[',
CONCAT_WS('&',FAUDIT_SUB_INDEX, FAUDIT_SRC_FIELD,FAUDIT_DST_FIELD,FAUDIT_ISKEY, FAUDIT_OPT),
']') ) sub_desc
from MIG_AUDITF_SUB
group by faudit_main_id