多对一处理

导入依赖

<dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.17</version></dependency><dependency><groupId>org.mybatis</groupId><artifactId>mybatis</artifactId><version>3.5.2</version></dependency><dependency><groupId>junit</groupId><artifactId>junit</artifactId><version>4.12</version><scope>test</scope></dependency></dependencies><build><resources><resource><directory>src/main/java</directory><includes><include>**/*.xml</include><include>**/*.properties</include></includes><filtering>true</filtering></resource></resources></build>

<dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>1.18.10</version></dependency>

pojo层

import lombok.Data;@Data
public class Student {private int id;private String name;//学生关联一个老师private Teacher teacher;}

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

dao层

public interface StudentMapper {//查询所有的学生信息以及对应的老师信息public List<Student> getStudent();
}

public interface TeacherMapper {@Select("select * from teacher where id = #{tid}")Teacher getTeacher(@Param("tid") int id);
}

MybatisUtils

public class MybatisUtils {private static SqlSessionFactory sqlSessionFactory;static{//使用mybatis第一步:获取sqlSessionFactory对象try {String resource="mybatis-config.xml";InputStream inputStream = null;inputStream = Resources.getResourceAsStream(resource);sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);} catch (IOException e) {e.printStackTrace();}}//有了sqlSeesionFactory之后，就可以从factory中获取sqlSession实例了//sqlSession完全包含了面向数据库执行SQL命令所需的所有方法public static SqlSession getSqlSession(){return sqlSessionFactory.openSession();}
}

方法一（按照查询嵌套处理）

StudentMapper.xml

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.zhbit.dao.StudentMapper"><!-- 方法一：按照查询嵌套处理1.查询所有的学生信息2.根据查询出来的学生的tid,寻找对应的老师 子查询--><select id="getStudent" resultMap="StudentTeacher" >select * from student</select><resultMap id="StudentTeacher" type="Student"><result property="id" column="id"/><result property="name" column="name"/><!--复杂的属性，需要单独处理对象使用：association集合:collection       --><association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/></resultMap><select id="getTeacher" resultType="com.zhbit.pojo.Teacher">select * from teacher where id = #{id}</select>
</mapper>

TeacherMapper.xml

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.zhbit.dao.TeacherMapper"></mapper>

测试类

public class MyTest {@Testpublic void testStudent(){try(SqlSession sqlSession = MybatisUtils.getSqlSession()){StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);List<Student> studentList = mapper.getStudent();for (Student student : studentList) {System.out.println(student);}}}
}

测试结果

方法二（按照结果嵌套查询）

StudentMapper.xml

<select id="getStudent2" resultMap="StudentTeacher2">select s.id as sid,s.name as sname,t.name as tname,s.tidfrom student s,teacher twhere s.tid = t.id;</select><resultMap id="StudentTeacher2" type="student"><result property="id" column="sid"/><result property="name" column="sname"/><association property="teacher" column="tname" javaType="teacher"><result property="id" column="tid"/><result property="name" column="tname"/></association></resultMap><!--方法一：按照查询嵌套处理1.查询所有的学生信息2.根据查询出来的学生的tid,寻找对应的老师 子查询--><!--<select id="getStudent" resultMap="StudentTeacher" >select * from student</select><resultMap id="StudentTeacher" type="Student"><result property="id" column="id"/><result property="name" column="name"/>&lt;!&ndash;复杂的属性，需要单独处理对象使用：association集合:collection       &ndash;&gt;<association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/></resultMap><select id="getTeacher" resultType="com.zhbit.pojo.Teacher">select * from teacher where id = #{id}</select>-->
</mapper>

测试类

public class MyTest {/*@Testpublic void getTeacher(){try(SqlSession sqlSession = MybatisUtils.getSqlSession()){TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);Teacher teacher = mapper.getTeacher(1);System.out.println(teacher);}}@Testpublic void testStudent(){try(SqlSession sqlSession = MybatisUtils.getSqlSession()){StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);List<Student> studentList = mapper.getStudent();for (Student student : studentList) {System.out.println(student);}}}*/@Testpublic void testStudent2(){try(SqlSession sqlSession = MybatisUtils.getSqlSession()){StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);List<Student> studentList = mapper.getStudent2();for (Student student : studentList) {System.out.println(student);}}}
}

测试结果

多对多处理

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Mybatis复杂查询环境

多对一处理

导入依赖

pojo层

dao层

MybatisUtils

方法一（按照查询嵌套处理）

StudentMapper.xml

测试类

测试结果

方法二（按照结果嵌套查询）

StudentMapper.xml

测试类

测试结果

多对多处理

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