MyBatis一对多嵌套查询的完整实例

前言

嵌套查询的实现原理为两次查询，比如产品表为主表，图片表为从表通过product_id字段与产品表id字段关联实现一对多，嵌套查询 首先查询 主表的数据 然后将主表id字段赋值给从表实体类中product_id 字段（productId）然后通过dao接口路径映射找到对应的MyBatis XMl文件SQL语句ID如：com.liao.dao.DImgMapper.selectDImgByProductId 进行子查询也就是第二次查询。然后返回数据

数据库建表语句和测试数据如下：

数据库版本为 MySQL 8.0

产品表

DROP TABLE IF EXISTS `d_product`;
CREATE TABLE `d_product` (
 `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '产品ID',
 `product_name` varchar(255) CHARACTER SET utf8 COLLATE utf8_bin NULL DEFAULT NULL COMMENT '产品名称',
 `product_introduction` text CHARACTER SET utf8 COLLATE utf8_bin NULL COMMENT '产品介绍',
 `product_category` int(11) NULL DEFAULT NULL COMMENT '产品ID',
 `product_status` int(1) NULL DEFAULT NULL COMMENT '产品状态',
 `create_time` datetime(0) NULL DEFAULT NULL COMMENT '创建时间',
 PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 18 CHARACTER SET = utf8 COLLATE = utf8_bin COMMENT = '产品表' ROW_FORMAT = Dynamic;

INSERT INTO `d_product` VALUES (1, '测试产品名称修改', '测试产品介绍修改', NULL, 1, '2020-02-02 12:40:06');
INSERT INTO `d_product` VALUES (2, '产品名称', '产品介绍', NULL, 1, '2020-03-02 18:15:07');
INSERT INTO `d_product` VALUES (3, 'bbb', 'bbb', NULL, 1, '2020-03-01 22:18:40');

图片表

DROP TABLE IF EXISTS `d_img`;
CREATE TABLE `d_img` (
 `id` int(11) NOT NULL AUTO_INCREMENT COMMENT 'id',
 `product_id` int(11) NULL DEFAULT NULL COMMENT '产品图片ID',
 `img` varchar(500) CHARACTER SET utf8 COLLATE utf8_bin NULL DEFAULT NULL COMMENT '图片',
 `create_time` datetime(0) NULL DEFAULT NULL COMMENT '创建时间',
 PRIMARY KEY (`id`) USING BTREE,
 INDEX `product_id`(`product_id`) USING BTREE,
 CONSTRAINT `d_img_ibfk_1` FOREIGN KEY (`product_id`) REFERENCES `d_product` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT
) ENGINE = InnoDB AUTO_INCREMENT = 86 CHARACTER SET = utf8 COLLATE = utf8_bin COMMENT = '产品图片' ROW_FORMAT = Dynamic;

INSERT INTO `d_img` VALUES (40, 1, '1491803633034_683819.jpg', '2020-03-03 17:21:20');
INSERT INTO `d_img` VALUES (40, 1, '1491803633034_683819.jpg', '2020-03-03 17:21:20');
INSERT INTO `d_img` VALUES (41, 1, '1568950881751_702421.jpg', '2020-03-03 17:21:20');

Java实体类：

// 将这个注解写在类上之后，就会忽略类中不存在的字段,否则可能会报错
@JsonIgnoreProperties(value = {"handler"})
/**
 * 
 * TODO: 产品类
 * @author LiAo
 * @date 2020/5/20 17:04
 */
public class DProduct {
 private Integer id;

 private String productName;

 private Integer productCategory;

 private Integer productStatus;

 private Date createTime;

 private String productIntroduction;

 private List<DImg> dImgs;  // 用于存放图片集合
 
 // .. get set toString
}
 
/**
 * 
 * TODO: 产品图片类
 * @author LiAo
 * @date 2020/5/20 17:05
 */
@JsonIgnoreProperties(value = {"handler"})
public class DImg {
 private Integer id;

 private Integer productId;

 private String img;

 private Date createTime;
 
 // .. get set toString
}

实体类创建好后要编写Dao接口 和Mapper XML了

持久层接口DAO：

/**
 *
 * TODO: 产品 Dao接口
 * @author LiAo
 * @date 2020/5/20 17:08
 */
public interface DProductMapper {
 /**
  * 产品图片一对多嵌套
  * @param record 查询条件
  * @return 返回参数
  */
 List<DProduct> productSelect(DProduct record);
}

产品MyBatis xml：

<!--映射的Dao接口类 可以通过这个路径找到先关的SQL语句和resultMap 映射-->
<mapper namespace="com.liao.dao.DProductMapper">
 <resultMap id="BaseResultMap" type="com.liao.entity.DProduct">
  <id column="id" property="id" jdbcType="INTEGER"/>
  <result column="product_name" property="productName" jdbcType="VARCHAR"/>
  <result column="product_category" property="productCategory" jdbcType="INTEGER"/>
  <result column="product_status" property="productStatus" jdbcType="INTEGER"/>
  <result column="create_time" property="createTime" jdbcType="TIMESTAMP"/>
  <result column="product_introduction" property="productIntroduction" jdbcType="LONGVARCHAR"/>
 </resultMap>
 <!--产品图片一对多查询映射-->
 <!--id：配置映射的名称-->
 <!--type：返回值类型 -->
 <!--extends：继承id为BaseResultMap的映射 -->
 <!--select：子查询所在的XML绑定的DAO接口路径.SQL语句id -->
 <!--column="{productId = id} productId：从表关联主表的实体类属性，作为子查询的条件 id：主表中被关联的数据库字段-->
 <resultMap id="dProductResultMap" type="com.liao.entity.DProduct" extends="BaseResultMap">
  <collection property="dImgs" fetchType="lazy"
     select="com.liao.dao.DImgMapper.selectDImgByProductId" column="{productId = id}"/>
 </resultMap>
 
 <!--查询语句-->
 <select id="productSelect" parameterType="com.liao.entity.DProduct" resultMap="dProductListMapSelect">
 select
 d.id,
 d.product_name,
 d.product_category,
 d.product_status,
 d.create_time,
 d.product_introduction
 from d_product d
 where 1 = 1
 <!-- 使用if标签拼接条件语句 实现动态SQL-->
 <if test="id != null and id != ''">
  and d.id = #{id}
 </if>
 <if test="productName != null and productName != ''">
  and d.product_name like concat(#{productName},'%')
 </if>
 <if test="productStatus != null and productStatus != ''">
  and d.product_status = #{productStatus}
 </if>
 <if test="createTime != null and createTime != ''">
  and d.create_time like concat(#{createTime},'%')
 </if>
 <if test="productIntroduction != null and productIntroduction != ''">
  and d.product_introduction like concat(#{productIntroduction},'%')
 </if>
</select>
</mapper>

图片MyBatis xml：

<mapper namespace="com.liao.dao.DImgMapper">
 <resultMap id="BaseResultMap" type="com.liao.entity.DImg">
  <id column="did" property="id" jdbcType="INTEGER"/>
  <result column="product_id" property="productId" jdbcType="INTEGER"/>
  <result column="img" property="img" jdbcType="VARCHAR"/>
  <result column="create_time" property="createTime" jdbcType="TIMESTAMP"/>
 </resultMap>
 <!--一对多嵌套查询 子查询语句-->
 <select id="selectDImgByProductId" resultMap="BaseResultMap">
  select i.id as did,
    i.product_id,
    i.img,
    i.create_time
  from d_img i
  where i.product_id = #{productId}
 </select>
</mapper>

测试查询结果

查询结果为一个产品对象里有若干个产品图片对象。

 			{
 				"id": 18,
    "productName": "产品添加图片上传测试",
    "productCategory": null,
    "productStatus": 1,
    "createTime": "2020-04-14T13:40:40.000+0000",
    "productIntroduction": "产品添加图片上传测试",
    "dImgs": [
     {
      "id": 92,
      "productId": 18,
      "img": "01.jpg",
      "createTime": "2020-04-26T02:33:04.000+0000"
     },
     {
      "id": 93,
      "productId": 18,
      "img": "1554103835292_610234.jpg",
      "createTime": "2020-04-26T02:33:04.000+0000"
     },
     {
      "id": 94,
      "productId": 18,
      "img": "1555484699771_582172.jpg",
      "createTime": "2020-04-26T02:33:04.000+0000"
     },
     {
      "id": 95,
      "productId": 18,
      "img": "1554103835292_610234.jpg",
      "createTime": "2020-04-26T02:33:04.000+0000"
     }
    ]
   },

总结

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