Mybatis多对一，一对多查询模式

• 1.多对一的处理
• • 按照查询嵌套处理
  • 按照结果嵌套处理
• 2.一对多的处理
• • 按照结果嵌套处理

1.多对一的处理

现数据库中存在teacher表和student表，其中student表中的tid为外键，连接teacher表中的id

实体类：

Student.java：

import lombok.Data;@Data
public class Student {private int id;private String name;private int age;private Teacher teacher;
}

Teacher.java：

import lombok.Data;@Data
public class Teacher {private int id;private String name;private String major;
}

按照查询嵌套处理

StudentMapper.xml：

具体的思路是，使用resultMap进行联合查询操作，封装到一个结果集中：

resultMap中使用association对对象类型进行处理👌

select * from studentselect * from teacher where id = #{id}

测试类：

public class StudentMapperTest {@Testpublic void test() {SqlSession sqlSession = MybatisUtils.getSqlSession();StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);List stuList = mapper.getStuList();for (Student student : stuList) {System.out.println(student);}sqlSession.close();}
}
--------------------------
Student(id=1, name=王伟, age=18, teacher=Teacher{id=1, name='鱼皮', major='语文'})
Student(id=2, name=李华, age=15, teacher=Teacher{id=1, name='鱼皮', major='语文'})
Student(id=3, name=皮皮, age=11, teacher=Teacher{id=1, name='鱼皮', major='语文'})
Student(id=4, name=树立, age=16, teacher=Teacher{id=2, name='大河', major='数学'})
Student(id=5, name=自立, age=17, teacher=Teacher{id=2, name='大河', major='数学'})

按照结果嵌套处理

StudentMapper.xml：

使用复杂的SQL语句直接进行连表查询：

select s.id sid , s.name sname , t.name tnamefrom student s , teacher twhere s.tid = t.id

测试类：

@Test
public void test() {SqlSession sqlSession = MybatisUtils.getSqlSession();StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);List stuListDou = mapper.getStuListDou();for (Student student : stuListDou) {System.out.println(student);}sqlSession.close();
}
---------------------------------------
Student(id=1, name=王伟, age=0, teacher=Teacher{id=1, name='鱼皮', major='鱼皮'})
Student(id=2, name=李华, age=0, teacher=Teacher{id=2, name='鱼皮', major='鱼皮'})
Student(id=3, name=皮皮, age=0, teacher=Teacher{id=3, name='鱼皮', major='鱼皮'})
Student(id=4, name=树立, age=0, teacher=Teacher{id=4, name='大河', major='大河'})
Student(id=5, name=自立, age=0, teacher=Teacher{id=5, name='大河', major='大河'})

2.一对多的处理

实体类：

Student.java：

import lombok.Data;@Data
public class Student {private int id;private String name;private int age;private int tid;
}

Teacher.java：

import lombok.Data;@Data
public class Teacher {private int id;private String name;private String major;private List students;
}

按照结果嵌套处理

开发接口类：

public interface TeacherMapper {Teacher getTeacherById (@Param("tid") int id);
}

TeacherMapper.xml：

select * from teacherselect s.id sid , s.name sname , t.name tname , t.id tidfrom student s , teacher twhere s.tid = t.id and t.id = #{tid}

测试类开发：

public class TeacherMapperTest {@Testpublic void testA() {SqlSession sqlSession = MybatisUtils.getSqlSession();TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);Teacher teacher = teacherMapper.getTeacherById(1);System.out.println(teacher);sqlSession.close();}
}

输出结果：

Teacher(id=1, name=鱼皮, major=null, students=[Student(id=1, name=王伟, age=0, tid=1), Student(id=2, name=李华, age=0, tid=1), Student(id=3, name=皮皮, age=0, tid=1)])