Mybatis--->第六节一对多和多对一

前情提要：Student和Teacher表，student，tid关联老师的tid

一对多

一个老师对应多个学生

老师和学生的接口，在两个文件里，太短了，就放在一起

public interface TeacherMapper {
    List<Teacher> selectTeacher(@Param("tid")int id);
    List<Teacher> selectTeacher2(@Param("tid")int id);
}



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

teacher 的map文件

<?xml version="1.0" encoding="GBK" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.xian.dao.TeacherMapper">
<!--   根据查询===也是推荐的方式==================================-->

<select id="selectTeacher" resultMap="Teacherss">
    select s.id sid,s.name sname,t.tid tid,t.tname tname
    from testdb.teacher t,testdb.student s
    where s.tid=t.tid and t.tid=#{tid}
</select>

    <resultMap id="Teacherss" type="Teacher">
    <result property="tid" column="tid"/>
    <result property="tname" column="tname"/>
<!-- collection 对应集合的意思，对应的Student是一个列表===对比多对一的association-->
    <collection property="studentlist" ofType="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <result property="tid" column="tid"/>
    </collection>
    </resultMap>
<!--    =============================================-->
    <select id="selectTeacher2" resultMap="Teacher2">
        select * from testdb.teacher
    </select>
    <resultMap id="Teacher2" type="Teacher">
        <collection property="studentlist" javaType="ArrayList" ofType="Student" select="getStudent" column="tid"/>
    </resultMap>
    <select id="getStudent" resultType="Student">
        select * from testdb.student where tid=#{tid}
    </select>
</mapper>

 

关联：association

集合：collection

Java type  和  of type

javatype 指定实体类中属性的类型

of Type　用来指定，集合、List的泛型

 

多对一

多个学生对应一个老师

接口文件和上面的一样不重复了，直接上代码

<?xml version="1.0" encoding="GBK" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.xian.dao.StudentMapper">
<!--    多对一 子查询，联表查询-->
<!--    =================================================-->
<!--根据结果-->

    <select id="selectStudent2" resultMap="student2">
        select s.id sid,s.name sname,t.tname tname from student s,teacher t where s.tid=t.tid
    </select>
    <resultMap id="student2" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

<!--    根据查询-->
<!--    =============================================-->
    <resultMap id="StudentMap" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <association property="teacher" column="tid" javaType="Teacher" select="selectTeacher"/>
    </resultMap>


    <select id="selectStudent" resultMap="StudentMap">
        select * from student
    </select>
    <select id="selectTeacher" resultType="com.xian.pojo.Teacher">
        select * from teacher where tid=#{tid}
    </select>
</mapper>