Mybatis：⼀对⼀查询

水深无声 2024-03-23 13:10 35阅读 0赞

### 需求 ###

查询⼀个订单，与此同时查询出该订单所属的⽤户

### Bean ###

public class Order {
    
        private Integer id;
        private String orderTime;
        private Double total;
    
        // 表明该订单属于哪个用户
        private User user;
    }
    
    public class User implements Serializable {
    
        private Integer id;
        private String username;
    }

### 第一种方式XML ###

#### Mapper ####

public interface IOrderMapper {
    
        //查询订单的同时还查询该订单所属的用户
        public List<Order> findOrderAndUser();
    }

#### XML ####

<resultMap id="orderMap" type="com.wuxian.pojo.Order">
            <result property="id" column="id"></result>
            <result property="orderTime" column="orderTime"></result>
            <result property="total" column="total"></result>
    
            <association property="user" javaType="com.wuxianu.pojo.User">
                <result property="id" column="uid"></result>
                <result property="username" column="username"></result>
            </association>
        </resultMap>
    
        
        <select id="findOrderAndUser" resultMap="orderMap">
            select * from orders o,user u where o.uid = u.id
        </select>

### 第二种方式注解 ###

#### Mapper ####

public interface IOrderMapper {
    
        //查询订单的同时还查询该订单所属的用户
        @Results({
                @Result(property = "id",column = "id"),
                @Result(property = "orderTime",column = "orderTime"),
                @Result(property = "total",column = "total"),
                @Result(property = "user",column = "uid",javaType = User.class,
                        one=@One(select = "com.lagou.mapper.IUserMapper.findUserById"))
        })
        @Select("select * from orders")
        public List<Order> findOrderAndUser();
    }
    
    
    public interface IUserMapper {
    
        //根据id查询用户
        @Options(useCache = true)
        @Select({"select * from user where id = #{id}"})
        public User findUserById(Integer id);
    }