feat(class): 删除班级时验证是否存在学生

- 新增StudentDOMapper接口方法selectStudentCountByClassId，用于查询班级下学生数量
- 在ClassServiceImpl中注入StudentDOMapper
- 删除班级时先判断班级下是否存在学生，若存在则抛出异常防止删除
- 更新StudentDOMapper.xml，添加对应的SQL查询语句selectStudentCountByClassId

enlish-service/src/main/java/com/yinlihupo/enlish/service/domain/mapper/StudentDOMapper.java

@@ -19,4 +19,6 @@ public interface StudentDOMapper {
 
     // 逻辑删除
     void deleteById(Integer id);
+
+    int selectStudentCountByClassId(@Param("classId") Integer classId);
 }

enlish-service/src/main/java/com/yinlihupo/enlish/service/service/classs/ClassServiceImpl.java

@@ -6,6 +6,7 @@ import com.yinlihupo.enlish.service.domain.dataobject.GradeDO;
 import com.yinlihupo.enlish.service.domain.mapper.ClassDOMapper;
 import com.yinlihupo.enlish.service.domain.mapper.GradeClassDOMapper;
 import com.yinlihupo.enlish.service.domain.mapper.GradeDOMapper;
+import com.yinlihupo.enlish.service.domain.mapper.StudentDOMapper;
 import com.yinlihupo.enlish.service.service.ClassService;
 import jakarta.annotation.Resource;
 import org.springframework.stereotype.Service;
@@ -24,6 +25,8 @@ public class ClassServiceImpl implements ClassService {
     private GradeClassDOMapper gradeClassDOMapper;
     @Resource
     private GradeDOMapper gradeDOMapper;
+    @Resource
+    private StudentDOMapper studentDOMapper;
 
     @Override
     public ClassDO findClassById(Integer id) {
@@ -82,6 +85,10 @@ public class ClassServiceImpl implements ClassService {
 
     @Override
     public void deleteClass(Integer classId) {
+        int selectStudentCountByClassId = studentDOMapper.selectStudentCountByClassId(classId);
+        if (selectStudentCountByClassId > 0) {
+            throw new RuntimeException("该班级下有学生，请先删除该班级下的学生");
+        }
         classDOMapper.delete(classId);
     }
 }

enlish-service/src/main/resources/mapper/StudentDOMapper.xml

@@ -61,4 +61,11 @@
         set is_deleted = 1
         where id = #{id}
     </update>
+
+    <select id="selectStudentCountByClassId" resultType="java.lang.Integer">
+        select count(1)
+        from student
+        where class_id = #{classId}
+            and is_deleted = 0
+    </select>
 </mapper>