Mybatis进阶学习笔记——关系查询——一对一查询

用户和订单的需求

通过查询订单，查询用户，就是一对一查询

（1）自定义JavaBean（常用，推荐使用）

     <select id="queryOrderUser" resultType="OrderUser">
         SELECT o.id,o.user_id,u.name,o.productname
         FROM t_order o LEFT JOIN t_user u ON o.user_id= u.id
     </select>

OrderUser.java：

 package cn.sm1234.domain;
 
 public class OrderUser extends Order {
 
     //Order中没有name
     private String name;
 
     public String getName() {
         return name;
     }
 
     public void setName(String name) {
         this.name = name;
     }
 
 }

     @Test
     public void test1() {
         SqlSession sqlSession = SessionUtils.getSession();
         // getMapper(): 返回指定接口的动态代理的实现类对象
         OrderDao dao = sqlSession.getMapper(OrderDao.class);
         List<OrderUser> list = dao.queryOrderUser();
         for (OrderUser orderUser : list) {
             System.out.println(orderUser.getName()+"-"+orderUser.getProductname());
         }
         sqlSession.commit();
         sqlSession.close();
     }

OrderDao.java：

 public interface OrderDao {
 
     /**
      * 查询订单，并且关联查询用户信息
      */
     public List<OrderUser> queryOrderUser();
 }

（2）ResultMap封装

     <!-- type="实体类" -->
     <resultMap type="Order" id="OrderUserResultMap">
         <id column="id" property="id"/>
         <result column="productname" property="productname"/>
         <result column="orderno" property="orderno"/>
         <!-- user_id封装到User中 -->
         <!-- 关联属性
             property：关联属性名称（Order类中的user）
             javaType：类型（User （即User user））
         -->
         <association property="user" javaType="User">
             <!-- column中的字段名称应该为当前查询结果的字段名称 -->
             <id column="user_id" property="id"/>
             <result column="name" property="name"/>
             <result column="password" property="password"/>
         </association>
 
     </resultMap>
 
     <select id="queryOrderUserResultMap" resultMap="OrderUserResultMap">
         SELECT o.id,o.user_id,u.name,o.productname
         FROM t_order o LEFT JOIN t_user u ON o.user_id= u.id
     </select>

Order.java：

 package cn.sm1234.domain;
 
 public class Order {
 
     private Integer id;
     private String productname;
     private String orderno;
     private Integer userId;
 
     private User user;
 
     public Integer getId() {
         return id;
     }
     public void setId(Integer id) {
         this.id = id;
     }
     public String getProductname() {
         return productname;
     }
     public void setProductname(String productname) {
         this.productname = productname;
     }
     public String getOrderno() {
         return orderno;
     }
     public void setOrderno(String orderno) {
         this.orderno = orderno;
     }
     public Integer getUserId() {
         return userId;
     }
     public void setUserId(Integer userId) {
         this.userId = userId;
     }
     public User getUser() {
         return user;
     }
     public void setUser(User user) {
         this.user = user;
     }
 
 }

     /**
      * 查询订单，查询用户（ResultMap）
      * @return
      */
     public List<Order> queryOrderUserResultMap();

     @Test
     public void test2() {
         SqlSession sqlSession = SessionUtils.getSession();
         // getMapper(): 返回指定接口的动态代理的实现类对象
         OrderDao dao = sqlSession.getMapper(OrderDao.class);
         List<Order> list = dao.queryOrderUserResultMap();
         for (Order order : list) {
             System.out.println(order.getProductname()+"-"+order.getUser().getName());
         }
         sqlSession.commit();
         sqlSession.close();
     }