技術標籤：Mybatis

環境搭建

1匯入lombok
2.新建實體類 Teacher Student
3.建立Mapper介面
4.建立Mapper.xml
5.在核心配置檔案中繫結介面
6.測實是否能夠執行

多對一

實體類

 private int id;
    private String name;
    //學生需要關聯一個老師
    private Teacher teacher;

private int id;
    private String name;

按照查詢巢狀處理

<!--
    思路：
        1.查詢所有的學生資訊
        2.根據查詢出來的學生的tid，尋找對應的老師
-->
    <select id="getStudent" resultMap="StudentTeacher">
        select * from student
    </select>

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

    <resultMap id="StudentTeacher" type="student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--複雜的屬性需要單獨處理    物件：association 集合：collection-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
 

按結果巢狀處理

  <select id="getStudent2" resultMap="StudentTeacher2">
        select s.id sid,s.name sname,t.name tname
        from student s,teacher t
        where s.tid=t.id
    </select>

    <resultMap id="StudentTeacher2" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>
 

回顧Mysql多對一查詢
1.子查詢
2.聯表查詢

一對多

按照結果巢狀處理

<!--    按結果巢狀查詢-->
    <select id="getTeacher" resultMap="TeacherStudent">
    select s.id sid ,s.name sname,t.name tname,t.id tid
    from teacher t,student s
    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"/>
<!--        javatype=“”指定屬性的型別
               集合中的泛型資訊，我們使用ofType-->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
 

按照查詢巢狀處理

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

    <resultMap id="TeacherStudent2" 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>

1.關聯-association【多對一】
2.集合-collection【一對多】

1.JavaType 用來指定實體類中屬性的型別
2.ofType 用來指定對映到List或者集合中的pojo型別，泛式中的約束型別

保證sql的可讀性 儘量保證能夠通俗易懂
注意一對多和多對一中，屬性名和欄位的問題