版权声明:java洪君 https://blog.csdn.net/qq_43532342/article/details/89138974
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)