String sql = "select power from teacher,student,admin where id= ? and pass= ? ";
https://blog.csdn.net/syyyyyyyyyyyyyyh/article/details/106467348
sql少了关联(外键)条件，比如根据学生id进行关联，类似如下，多表建议设置别名
select power from teacher as t,student as s,admin as a where id= ? and pass= ?
and t.student_id = s.student_id
and s.student_id = a.student_id