mysql 左连接一直报列重复问题

SELECT tgr.grade_id , tgr.name FROM t_grade tgr LEFT JOIN (
#查询所有可以升班的班级
SELECT tc.class_id ,tc.comp_info_id ,tg.sort,CAST(tc.graduation_year as SIGNED) graduation_year ,tg.sort from t_class tc LEFT JOIN t_grade tg on
tg.grade_id=tc.grade WHERE tc.status = 1 AND tg.sort is not NULL AND tc.graduation_year> DATE_FORMAT(NOW(), '%Y') ORDER BY comp_info_Id

 ) AA  on     tgr.sort >  AA.sort AND   tgr.comp_info_id = AA.comp_info_id


            错误：> 1060 - Duplicate column name 'sort'

时间: 0.001s

SELECT tc.class_id ,tc.comp_info_id ,**tg.sort**,CAST(tc.graduation_year as SIGNED) graduation_year ,**tg.sort** from t_class tc LEFT JOIN t_grade tg on

你这句话里 写了 两遍 tg.sort