MyBatis复杂查询环境，多对一、一对多处理

复杂环境搭建

①创建SQL表

CREATE TABLE `teacher` (
    `id` INT(10) NOT NULL,
    `name` VARCHAR(30) DEFAULT NULL,
    PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO teacher(`id`, `name`) VALUEs (1, 'qin');

CREATE TABLE `student` (
    `id` INT(10) NOT NULL,
    `name` VARCHAR(30) DEFAULT NULL,
    `tid` INT(10) DEFAULT NULL,
    PRIMARY KEY (`id`),
    KEY `ftkid` (`tid`),
    CONSTRAINT `ftkid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;


INSERT INTO student(`id`, `name`, `tid`) VALUEs (1, 'yi', '1');
INSERT INTO student(`id`, `name`, `tid`) VALUEs (2, 'er', '1');
INSERT INTO student(`id`, `name`, `tid`) VALUEs (3, 'san', '1');
INSERT INTO student(`id`, `name`, `tid`) VALUEs (4, 'si', '1');
INSERT INTO student(`id`, `name`, `tid`) VALUEs (5, 'wu', '1');

建立老师、同学表，同学的tid对应老师外键id

②创建表对应实体类Student和Teacher

③建立Mapper接口

④建立Mapper对应xml文件

⑤核心配置中注册Mapper接口文件

多对一处理

多个学生由一个老师教学，查询出所有学生以及每个学生对应老师的信息。

嵌套子查询

<!-- ①查询所有学生
     ②依据tid查询对应老师
-->
    <resultMap id="studentTeacher" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
<!--        复杂属性需要单独处理
            对象使用association
            集合使用collection
-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>


    <select id="getStudent" resultMap="studentTeacher">
        select * from student s, teacher t where s.tid = t.id;
    </select>

    <select id="getTeacher" resultType="Teacher">
        select * from teacher where id=#{tid};
    </select>

因为学生对象中有Teacher复杂成员，用resultMap对其处理，该对象由tid属性再次执行查询命令。

按结果嵌套

    <select id="getStudent" resultMap="StudentTeacher">
        select s.id sid,s.name sname,t.name tname,t.id tid from student s, teacher t where s.tid = t.id;
    </select>

    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="id" column="tid"/>
            <result property="name" column="tname"/>
         </association>
    </resultMap>

该方式就是依据查询结果，用resultMap一一对应设置进去。

一对多处理

一个老师教学多个学生，对于老师就是一对多关系，老师实体类用List集合存储学生

结果嵌套处理

    <select id="getTeacher" 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>

注意使用collection中，获得集合泛型类型时，需要用ofType="Student"

嵌套查询处理

    <select id="getTeacher" resultMap="TeacherStudent">
        select * from teacher where id = #{tid}
    </select>
    <resultMap id="TeacherStudent" type="Teacher">
        <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
    </resultMap>

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

小结

①关联 association：对照复杂对象，多对一
②集合 collection：对照集合，一对多
③javaType：指定实体类中属性类型
④ofType：指定集合泛型类属性类型