Mybatis中的collection和association

https://mybatis.org/mybatis-3/zh/index.html

---

1 創建關係

CREATE TABLE tb_clazz
(
    `id`   INT AUTO_INCREMENT,
    `code` VARCHAR(64),
    `name` VARCHAR(12),
    PRIMARY KEY (`id`)
);
CREATE TABLE tb_student
(
    `id`       INT AUTO_INCREMENT,
    `clazz_id` INT,
    `name`     VARCHAR(20),
    `sex`      CHAR(2),
    `age`      TINYINT,
    PRIMARY KEY (`id`),
    FOREIGN KEY (`clazz_id`) REFERENCES tb_clazz (`id`)
);

INSERT INTO tb_clazz(`id`, `code`, `name`)
VALUES (1, '02131704', '軟工四班');
INSERT INTO tb_student(`name`, `clazz_id`, `sex`, `age`)
VALUES ('jack', 1, '男', 23),
       ('rose', 1, '女', 18),
       ('tom', 1, '男', 21),
       ('alice', 1, '女', 20);

@Data
public class Clazz {
    /**
     * 班級id，主鍵
     */
    @TableId(type = IdType.AUTO)
    private Integer id;
    /**
     * 班級編號
     */
    private String code;
    /**
     * 班級名稱
     */
    private String name;
    /**
     * 班級和學生是一對多的關係，即一個班級可以有多個學生
     */
    private List<Student> students;
}

@Data
public class Student {
    /**
     * 學生id，主鍵
     */
    @TableId(type = IdType.AUTO)
    private Integer id;
    /**
     * 姓名
     */
    private String name;
    /**
     * 性別
     */
    private String sex;
    /**
     * 年齡
     */
    private Integer age;
    /**
     * 學生和班級是多對一的關係，即一個學生只屬於一個班級
     */
    private Clazz clazz;
}

---

2 collection定義一對多關係

查詢指定班級的所有學生

<!--
    ClazzMapper.xml
-->
<resultMap type="cn.yuanyu.mybatis.entity.Clazz" id="clazzResultMap">
    <id property="id" column="id"/>
    <result property="code" column="code"/>
    <result property="name" column="name"/>
    <!--
         一對多關聯映射
         fetchType="lazy"表示懶加載
         ClazzMapper.xml
     -->
    <collection property="students" javaType="ArrayList"
                column="id" ofType="cn.yuanyu.mybatis.entity.Student"
                select="cn.yuanyu.mybatis.mapper.StudentMapper.selectStudentByClazzId"
                fetchType="lazy">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="sex" column="sex"/>
        <result property="age" column="age"/>
    </collection>
</resultMap>
<select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
    SELECT *
    FROM tb_clazz
    WHERE id = #{id}
</select>

<!-- 
    根據班級id查詢學生信息
    StudentMapper.xml
-->
<select id="selectStudentByClazzId" resultType="cn.yuanyu.mybatis.entity.Student">
    SELECT *
    FROM tb_student
    WHERE clazz_id = #{id}
</select>

 

<!--
    ClazzMapper.xml
-->
<resultMap type="cn.yuanyu.mybatis.entity.Clazz" id="clazzResultMap">
    <id property="id" column="cId"/>
    <result property="code" column="cCode"/>
    <result property="name" column="cName"/>
    <!--  -->
    <collection property="students" ofType="cn.yuanyu.mybatis.entity.Student">
        <id property="id" column="sId"/>
        <result property="name" column="sName"/>
        <result property="sex" column="sSex"/>
        <result property="age" column="sAge"/>
    </collection>
</resultMap>
<select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
    SELECT c.id   cId,
           c.code cCode,
           c.name cName,
           s.id   sId,
           s.name sName,
           s.sex  sSex,
           s.age  sAge
    FROM tb_clazz c
             JOIN tb_student s ON c.id = s.clazz_id and c.id = #{id}
</select>

public interface ClazzMapper {
    /**
     * 根據id查詢班級信息，以及所有學生
     *
     * @param id 班級id
     * @return 班級
     */
    Clazz selectClazzById(Integer id);
}

@Test
public void selectClazzById() {
    Clazz clazz = clazzMapper.selectClazzById(1);
    System.out.println("\n\n---------------------分割線---------------------\n\n");
    //List<Student> students = clazz.getStudents(); //這裏纔開始執行查詢

---

3 association定義多對一關係

查詢用戶信息，包括所在的班級信息

<!--
    StudentMapper.xml
-->
<resultMap type="cn.yuanyu.mybatis.entity.Student" id="studentResultMap">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="sex" column="sex"/>
    <result property="age" column="age"/>
    <!--
        多對一關聯映射
    -->
    <association property="clazz"
                 select="cn.yuanyu.mybatis.mapper.ClazzMapper.getClazzById"
                 column="clazz_id">
    </association>
</resultMap>
<!-- 根據id查詢學生信息 -->
<select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
    SELECT *
    FROM tb_student
    WHERE id = #{id}
</select>

<!--
    ClazzMapper.xml
-->
<select id="getClazzById" resultType="cn.yuanyu.mybatis.entity.Clazz">
    SELECT *
    FROM tb_clazz
    WHERE id = #{id}
</select>

 

​<!--
    StudentMapper.xml
 -->
<resultMap type="cn.yuanyu.mybatis.entity.Student" id="studentResultMap">
    <id property="id" column="sId"/>
    <result property="name" column="sName"/>
    <result property="sex" column="sSex"/>
    <result property="age" column="sAge"/>
    <!--
        多對一關聯映射
    -->
    <association property="clazz" javaType="cn.yuanyu.mybatis.entity.Clazz">
        <id property="id" column="cId"/>
        <result property="code" column="cCode"/>
        <result property="name" column="cName"/>
    </association>
</resultMap>
<!-- 根據id查詢學生信息，多表連接 -->
<select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
    SELECT s.id   sId,
           s.name sName,
           s.sex  sSex,
           s.age  sAge,
           c.id   cId,
           c.code cCode,
           c.name cName
    FROM tb_student s,
         tb_clazz c
    WHERE s.clazz_id = c.id
      AND s.id = #{id}
</select>

​

public interface StudentMapper {
    /**
     * 根據id查詢學生信息，班級信息也要查詢出來
     *
     * @param id 學生id
     * @return 學生
     */
    Student selectStudentById(Integer id);
}

@Test
public void selectStudentById() {
    Student student = studentMapper.selectStudentById(1);

https://blog.csdn.net/le_Jie/article/details/93931502