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如何在查询学生信息时包含完整的班级信息？

在数据库设计中，学生表和班级表之间存在关联关系。为了查询学生信息时包含完整的班级信息，可以采用以下几种方法：

1. 业务装配方式

思路：对学生表、班级表分别单表查询，在业务包中调用单表查询SQL，拼出结果。

实现：

• 学生实体类：

public class Student {
    int id;
    String name;
    int age;
    String gender;
    int cid;
    Clazz clazz;
    public Student() {
        // TODO Auto-generated constructor stub
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }
    public int getCid() {
        return cid;
    }
    public void setCid(int cid) {
        this.cid = cid;
    }
    public Clazz getClazz() {
        return clazz;
    }
    public void setClazz(Clazz clazz) {
        this.clazz = clazz;
    }
    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + ", age=" + age + ", gender=" + gender + ", cid=" + cid
                + ", clazz=" + clazz + "]";
    }
}

• 班级实体类：

public class Clazz {
    private int id;
    private String name;
    private String room;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getRoom() {
        return room;
    }
    public void setRoom(String room) {
        this.room = room;
    }
    @Override
    public String toString() {
        return "Clazz [id=" + id + ", name=" + name + ", room=" + room + "]";
    }
    public Clazz() {
        super();
    }
}

• 业务层实现类：

public class StudentServiceImpl implements StudentService {
    @Override
    public List
   
     selAll() {
        Session session = MyBatisUtil.getSession();
        StudentMapper stuMapper = session.getMapper(StudentMapper.class);
        ClazzMapper clsMapper = session.getMapper(ClazzMapper.class);
        List
    
      list = new ArrayList
     
      ();
        list = stuMapper.selAll();
        for (Student s : list) {
            System.out.println(s);
        }
        for (Student s : list) {
            s.setClazz(clsMapper.selClazzById(s.getCid()));
        }
        for (Student s : list) {
            System.out.println(s);
        }
        return list;
    }
}
     
    
   

• 持久层接口：

public interface StudentMapper {
    List
   
     selAll();
}
public interface ClazzMapper {
    Clazz selClazzById(int id);
}
   

• 持久层XML文件：

   
    
    id,name,age,gender,cid
   
    select from t_student

    select from t_class where id = #{id}

2. n+1方式

思路：使用MyBatis提供的resultMap标签，本质是Mybatis根据resultMap和selAll接口生成装配的Java代码。

实现：

• 业务层实现类：

public class StudentServiceImpl implements StudentService {
    @Override
    public List
   
     selAll2() {
        Session session = MyBatisUtil.getSession();
        StudentMapper stuMapper = session.getMapper(StudentMapper.class);
        List
    
      list = new ArrayList
     
      ();
        list = stuMapper.selAll2();
        for (Student s : list) {
            System.out.println(s);
        }
        for (Student s : list) {
            s.setClazz(clsMapper.selClazzById(s.getCid()));
        }
        for (Student s : list) {
            System.out.println(s);
        }
        return list;
    }
}
     
    
   

• 持久层接口：

public interface StudentMapper {
    List
   
     selAll2();
}
   

• 持久层XML文件：

   
    
    
    
    
    
    
   
    select from t_student

3. 使用join语句

思路：使用MyBatis的join语句结合resultMap标签，通过一次查询获取所有需要的信息。

实现：

• 业务层实现类：

public class StudentServiceImpl implements StudentService {
    @Override
    public List
   
     selAll3() {
        Session session = MyBatisUtil.getSession();
        StudentMapper stuMapper = session.getMapper(StudentMapper.class);
        List
    
      list = new ArrayList
     
      ();
        list = stuMapper.selAll3();
        for (Student s : list) {
            System.out.println(s);
        }
        return list;
    }
}
     
    
   

• 持久层接口：

public interface StudentMapper {
    List
   
     selAll3();
}
   

• 持久层XML文件：

   
    
    
    
    
    
    
    
    
    
    
    
    
        
     
        
     
        
     
    
    
   
    select s.id sid,s.name sname,s.age age,s.gender gender,s.cid scid,
           c.id cid,c.name cname,c.room room from t_student s
    left join t_class c on s.cid = c.id

4. 使用join语句，和resultType

思路：利用MyBatis的自动映射和``机制，代替resultMap。

实现：

• 业务层实现类：

public class StudentServiceImpl implements StudentService {
    @Override
    public List
   
     selAll4() {
        Session session = MyBatisUtil.getSession();
        StudentMapper stuMapper = session.getMapper(StudentMapper.class);
        List
    
      list = new ArrayList
     
      ();
        list = stuMapper.selAll4();
        for (Student s : list) {
            System.out.println(s);
        }
        return list;
    }
}
     
    
   

• 持久层接口：

public interface StudentMapper {
    List
   
     selAll4();
}
   

• 持久层XML文件：

    select s.id id,s.name name,s.age age,s.gender gender,s.cid cid,
           c.id `clazz.id`,c.name `clazz.name`,c.room `clazz.room` from t_student s
    left join t_class c on s.cid = c.id

5. 使用注解

思路：使用Mybatis提供的注解代替XML文件（这种方法，不需要StudentMapper.xml）。

实现：

• 持久层接口：

public interface StudentMapper {
    @Select(value = {
        "select * from t_student"
    })
    @Results(value = {
        @Result(column = "id", property = "id", id = true),
        @Result(column = "name", property = "name"),
        @Result(column = "age", property = "age"),
        @Result(column = "gender", property = "gender"),
        @Result(column = "cid", property = "cid"),
        @Result(property = "clazz", one = @One(select = "com.whl.mapper.ClazzMapper.selById"), column = "cid")
    })
    List
   
     selAll();
}
public interface ClazzMapper {
    @Select(value = {
        "select * from t_class where id = #{0}"
    })
    Clazz selById(int id);
}
   

总结

以上是查询学生信息时包含完整班级信息的几种方法。可以根据实际需求选择合适的方式。在实际项目中，join语句结合resultMap标签是一种高效且常用的方法。