GROUP_CONCAT(DISTINCT ct.class_id)

SELECT  GROUP_CONCAT(DISTINCT ct.class_id)  FROM t_class_teacher ct

import java.util.*;

List<String> stringList = Arrays.asList(vo.getClassNum().split(","));
                vo.setClassNum(new HashSet<>(stringList).size() + "");

LENGTH(str)

SUBSTR(str FROM pos FOR len)