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👌
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "https://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="top.imustctf.dao.StudentMapper"> <select id="getStuList" resultMap="StudentTeacher"> select * from student </select> <resultMap id="StudentTeacher" type="Student"> <result column="id" property="id"/> <result column="name" property="name"/> <result column="age" property="age"/> <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/> </resultMap> <select id="getTeacher" resultType="Teacher"> select * from teacher where id = #{id} </select> </mapper>
测试类：
public class StudentMapperTest { @Test public void test() { SqlSession sqlSession = MybatisUtils.getSqlSession(); StudentMapper mapper = sqlSession.getMapper(StudentMapper.class); List<Student> 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 id="getStuListDou" resultMap="StudentTeacherDou"> select s.id sid , s.name sname , t.name tname from student s , teacher t where s.tid = t.id </select> <resultMap id="StudentTeacherDou" type="Student"> <result property="id" column="sid"/> <result property="name" column="sname"/> <association property="teacher" javaType="Teacher"> <result property="name" column="tname"/> </association> </resultMap>
测试类：
@Test public void test() { SqlSession sqlSession = MybatisUtils.getSqlSession(); StudentMapper mapper = sqlSession.getMapper(StudentMapper.class); List<Student> 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<Student> 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：
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "https://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="top.imustctf.dao.TeacherMapper"> <select id="getTeacherList" resultType="Teacher"> select * from teacher </select> <select id="getTeacherById" resultMap="TeacherStudent"> select s.id sid , s.name sname , t.name tname , t.id tid from student s , teacher t where s.tid = t.id and t.id = #{tid} </select> <resultMap id="TeacherStudent" type="Teacher"> <result property="id" column="tid"/> <result property="name" column="tname"/> <collection property="students" ofType="Student"> <result property="id" column="sid"/> <result property="name" column="sname"/> <result property="tid" column="tid"/> </collection> </resultMap> </mapper>
测试类开发：
public class TeacherMapperTest { @Test public 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,
