8.1、association 关联 【多对一】

create table student
(
    id   int          not null
        primary key,
    name varchar(255) null,
    tid  int          not null,
    constraint student_ibfk_1
        foreign key (tid) references teacher (id)
);
create index tid
    on student (tid);

create table teacher
(
    id   int          not null comment 'tid'
        primary key,
    name varchar(255) null
);

List<Student> getStudent();

List<Student> getStudent2();

<!--==================方式一=================-->
<select id="getStudent" resultMap="TeacherStudent">
    select * from mybatis.student
</select>
<resultMap id="TeacherStudent" type="Student">
    <result property="id" column="id"/>
    <result property="name" column="name"/>

    <association property="teacher" column="tid"
                 javaType="com.yan.pojo.Teacher" select="getTeacher"/>
</resultMap>
<select id="getTeacher" resultType="Teacher">
    select * from mybatis.teacher where id = #{id}
</select>
<!--====================方式二==================-->
<resultMap id="TeacherStudent2" type="com.yan.pojo.Student">
    <result property="id" column="sid"/>
    <result property="name" column="sname"/>
    <association property="teacher" javaType="Teacher">
        <result property="name" column="tname"/>
    </association>
</resultMap>
<select id="getStudent2" resultMap="TeacherStudent2">
    select s.id as sid, s.name as sname,t.name as tname
    from mybatis.student s, mybatis.teacher t
    where s.tid = t.id
</select>

association 分步查询使用延迟加载

在分步查询的基础上，可以使用延迟加载来提升查询的效率，只需要在全局的Settings 中进行如下的配置:

<!-- 开启延迟加载-->
<setting name="lazyLoadingEnabled" value="true"/>
<!-- 设置加载的数据是按需还是全部-->
<setting name="aggressiveLazyLoading" value="false"/>

8.2、collection 集合 【一对多】

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

    private List<Student> students;
}

Teacher getTeacher(@Param("tid") int id);

1. 按照结果嵌套处理<resultMap id="TeacherStudent" type="com.yan.pojo.Teacher"> <result property="id" column="tid"/> <result property="name" column="tname"/> <collection property="students" ofType="Student"> <!--property: 关联的属性名 ofType: 集合中元素的类型--> <result property="id" column="sid"/> <result property="name" column="sname"/> <result property="tid" column="tid"/> </collection> </resultMap> <select id="getTeacher" resultMap="TeacherStudent"> 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 and t.id = #{tid} </select>
2. 按照查询嵌套处理

<resultMap id="TeacherStudent2" type="com.yan.pojo.Teacher">
    <collection property="students" ofType="com.yan.pojo.Student"
                select="getStudentByTeacherId" column="id">
    </collection>
</resultMap>

<select id="getStudentByTeacherId" resultType="Student">
    select * from mybatis.student where tid = #{tid}
</select>

<select id="getTeacher2" resultMap="TeacherStudent2">
    select * from mybatis.teacher where id = #{tid}
</select>

扩展: association 或collection 的fetchType 属性

1. 在<association> 和<collection>标签中都可以设置fetchType，指定本次查询是否要使用延迟加载。默认为fetchType=”lazy” ,如果本次的查询不想使用延迟加载，则可设置为fetchType=”eager”.
2. fetchType 可以灵活的设置查询是否需要使用延迟加载，而不需要因为某个查询不想使用延迟加载将全局的延迟加载设置关闭.