MyBatis 表关联查询 MyBatis中的表关联查询实现示例

 //(省去了get和set的方法)
 public class Goods {
  private Integer goodsId ;
  private String title ;
  private String subTitle ;
  private Float originalCost ;
  private Float currentPrice ;
  private Float discount ;
  private Integer isFreeDelivery ;
  private Integer categoryId ;
  //在一对多关系中，在一方添加多的一方的集合
  private List<GoodsDetail> goodsDetailLists ; 
}

<!--  OneToMany对象关联查询
    resultMap可用于说明一对多或者多对一的映射逻辑
    id 是resultMap属性引用的标志
    type 指向One的实体(Goods)
-->
  <resultMap id="rmGoods1" type="com.imooc.mybatis.entity.Goods">
<!--    映射goods对象的主键到goods_id字段-->
    <id column="goods_id" property="goodsId"></id>
<!--
    collection的含义是，在
    sql语句得到结果后，对所有Goods对象遍历得到goods_id字段值，
    并代入到goodsDetail命名空间的selectByGoodsId的sql中执行查询
    将得到的“商品详情”集合赋值给goodsDetailsList对象
-->
    <collection property="goodsDetailLists" select="goodsDetail.selectByGoodsId"
          column="goods_id"></collection>
  </resultMap>
  <select id="selectOneToMany" resultMap="rmGoods1">
    select * from t_goods limit 0 , 1
  </select>

<mapper namespace="goodsDetail">
  <select id="selectByGoodsId" parameterType="Integer"
      resultType="com.imooc.mybatis.entity.GoodsDetail">
    select * from t_goods_detail where goods_id = #{value}
  </select>
</mapper>

 //OneToMany
  @Test
  public void selectOneToMany(){
    SqlSession sqlSession = null ;
    try{
      sqlSession = MybatisUtils.openSession() ;
      List<Goods> list = sqlSession.selectList("goods.selectOneToMany");
      for (Goods g : list){
      //输出商品和该商品的详情信息数量
        System.out.println(g.getTitle() + ":" + g.getGoodsDetailLists().size());
      }
    }catch (Exception e){
      e.printStackTrace();
    }finally {
      MybatisUtils.closeSession(sqlSession);
    }
  }

public class GoodsDetail {
  private Integer gdId ;
  private Integer goodsId ;
  private String gdPicUrl ;
  private Integer gdOrder ;
  //多对一：在多的一方添加一的一方的实体
  private Goods goods ;
}

<!--  多对一关系-->
  <resultMap id="rmGoodsDetail" type="com.imooc.mybatis.entity.GoodsDetail">
    <id column="gd_id" property="gdId"></id>
    <result column="goods_id" property="goodsId"></result>
    <!-- goods.selectById 为goods.xml根据主键id查找goods信息。-->
    <association property="goods" select="goods.selectById" column="goods_id"></association>
  </resultMap>
  <select id="selectManyToOne" resultMap="rmGoodsDetail">
    select * from t_goods_detail limit 0 , 1
  </select>

/**
   * 多对一对象关联映射
   * */
  @Test
  public void selectManyToOne(){
    SqlSession sqlSession = null ;
    try{
      sqlSession = MybatisUtils.openSession() ;
      List<GoodsDetail> list = sqlSession.selectList("goodsDetail.selectManyToOne");
      for (GoodsDetail gd : list){
        System.out.println(gd.getGdPicUrl() + ":" + gd.getGoods().getTitle());
      }
    }catch (Exception e){
      e.printStackTrace();
    }finally {
      MybatisUtils.closeSession(sqlSession);
    }
  }