Mybatis中的一对多关联查询

有两张表：

学生表：

教师表：

其关联关系可表示为：

在一对多查询时（一个老师对多个学生），可想到方法：

1. 子查询
2. 关联查询

将所需的教室和学生类构建好后，将mybatis核心配置完成后(所需的类和核心配置文件在文章末尾)
StudentMapper.xml文件可进行查询：

方式一：

<!--    1.查询所有学生id-->
<!--    2.根据查询出来的学生的tid，寻找对应老师（子查询）-->
    <select id="getStudent" resultMap="StudentTeacher">
        select * from mybatis.student;
    </select>

    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"></result>
        <result property="name" column="name"></result>

        <association property="teacher" column="tid"  javaType="Teacher" select="getTeacher"></association>
    </resultMap>
    <select id="getTeacher" resultType="Teacher">
        select * from mybatis.teacher where id = #{id}
    </select>

方式二：

<!--    按照结果嵌套处理-->
<select id="getStudent2" resultMap="StudentTeacher2">
    select  s.id sid ,s.name sname, t.name tname, t.id tid from mybatis.student s,mybatis.teacher t where  s.tid =t.id
</select>

    <resultMap id="StudentTeacher2" type="Student">
        <result property="id" column="sid"></result>
        <result property="name" column="sname"></result>

        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"></result>
        </association>
    </resultMap>

注意：如果出现空则是老师id没有 查到，需要加上：
<result property="id" column="tid"></result>

测试结果：

Student(id=1, name=小明, teacher=Teacher(id=0, name=秦老师))
Student(id=2, name=小红, teacher=Teacher(id=0, name=秦老师))
Student(id=3, name=小张, teacher=Teacher(id=0, name=秦老师))
Student(id=4, name=小李, teacher=Teacher(id=0, name=秦老师))
Student(id=5, name=小王, teacher=Teacher(id=0, name=秦老师)

测试所需的资源

核心配置文件：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<!--核心配置文件-->
<configuration>

<!--    配置文件的实现-->
    <properties resource="db.properties" />

    <settings>
<!--        标准的日志 -->
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>

<!--    别名扫描包-->
    <typeAliases>
        <package name="com.tang.pojo"/>
    </typeAliases>


    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <package name="com.tang.dao" />
    </mappers>
</configuration>

核心封装类:

public class MybatisUtils {
    private static SqlSessionFactory sqlSessionFactory;

    static {
        try {
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
           sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSqlSession() {
        return  sqlSessionFactory.openSession();
    }
}

Student类：

@Data
public class Student {
    private  int id;
    private  String name;

    //学生需要关联一个老师
    private  Teacher teacher;
}

Teacher类：

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

TtudentMapper接口：

public interface TeacherMapper {

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

StudentMapper接口：

public interface StudentMapper {
    public List<Student> getStudent();
    public List<Student> getStudent2();
}

测试类：

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

    @Test
    public  void testStudent2(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> studentList = mapper.getStudent2();
        for (Student student : studentList) {
            System.out.println(student);
        }

        sqlSession.close();
    }
  }