多表关联查询

resultMap通常用于比较复杂的结果集映射(如：多表关联查询)的情况，使用步骤如下：

1. 显示定义标签映射结果集

   <resultMap id="userResultMap" type="User">
     <id property="id" column="user_id" />
     <result property="username" column="user_name"/>
     <result property="password" column="hashed_password"/>
   </resultMap>
   

2. 在查询语句中引用我们定义的resultMap：

   <select id="selectUsers" resultMap="userResultMap">
     select user_id, user_name, hashed_password
     from some_table
     where id = #{id}
   </select>
   

多表关联查询（级联查询）示例

1. 一对一查询
   jdbc.properties数据源

   jdbc.url=jdbc:mysql://localhost:3306/yanfa5
   jdbc.driver=com.mysql.jdbc.Driver
   jdbc.user=root
   jdbc.password=root
   jdbc.characterEncoding=utf8
   

   BookMapper.xml

   <?xml version="1.0" encoding="UTF-8" ?>
   <!DOCTYPE mapper
           PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
           "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
   
   <!-- 级联查询,一对一. -->
   <!-- namespace对应空Dao接口的全名 -->
   <mapper namespace="com.lanou3g.mybatis.dao.BookDao">
       <resultMap id="book" type="com.lanou3g.mybatis.bean.Book">
           <id property="id" column="id" />
           <result property="bname" column="bname" />
           <result property="author" column="author" />
           <result property="authorGender" column="author_gender" />
           <result property="price" column="price" />
           <result property="description" column="description" />
           <association property="bookType" javaType="com.lanou3g.mybatis.bean.BookType">
               <id property="id" column="id" />
               <result property="tname" column="tname" />
           </association>
       </resultMap>
       
       <select id="queryBook" resultMap="book">
           select b.*,bt.*,b.id,bt.id from book b,booktype bt where b.btype = bt.id;
       </select>
       
   </mapper>
   

   mybatis.xml

   <?xml version="1.0" encoding="UTF-8" ?>
   <!DOCTYPE configuration
           PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
           "http://mybatis.org/dtd/mybatis-3-config.dtd">
   <configuration>
       <properties resource="jdbc.properties" />
       <environments default="development">
           <environment id="development">
               <transactionManager type="JDBC"/>
               <dataSource type="POOLED">
                   <property name="driver" value="${jdbc.driver}"/>
                   <property name="url" value="${jdbc.url}"/>
                   <property name="username" value="${jdbc.user}"/>
                   <property name="password" value="${jdbc.password}"/>
               </dataSource>
           </environment>
       </environments>
       <mappers>
           <mapper resource="mapper/BookMapper.xml" />
       </mappers>
   </configuration>
   

   对应数据库表的java类
   注意：此处可以体现出一对一

   @Setter
   @Getter
   public class Book {
       private Integer id;
       private String bname;
       private BookType bookType;	// ————>一对一体现在此处<————
       private String author;
       private Integer authorGender;
       private Float price;
       private String description;
   
       @Override
       public String toString() {
           return "Book{" +
                   "id=" + id +
                   ", bname='" + bname + '\'' +
                   ", bookType=[" + bookType + ']' +
                   ", author='" + author + '\'' +
                   ", authorGender=" + authorGender +
                   ", price=" + price +
                   ", description='" + description + '\'' +
                   "}\n";
       }
   }
   
   
   @Getter
   @Setter
   public class BookType {
       private Integer id;
       private  String tname;
   
       @Override
       public String toString() {
           return "BookType{" +
                   "id=" + id +
                   ", tname='" + tname + '\'' +
                   "}\n";
       }
   }
   

   dao层接口（操作数据库）

   public interface BookDao {
       List<Book> queryBook();
   }
   

   程序运行入口

   public class App {
       public static void main(String[] args) throws IOException {
           //1、读入配置文件
           String confPath = "mybatis.xml";
           InputStream is = Resources.getResourceAsStream(confPath);
   
           //2、构建SqlSessionFactory(用于获取sqlSession)
           SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(is);
   
           //3、获取sqlSession对象（用于具体的RUID)
           SqlSession sqlSession = sessionFactory.openSession();
   
           //4、具体的RUID
           BookDao bookDao = sqlSession.getMapper(BookDao.class);
           System.out.println(bookDao.queryBook());
       }
   }
   

2. 一对多查询
   jdbc.properties数据源

   jdbc.url=jdbc:mysql://localhost:3306/yanfa5
   jdbc.driver=com.mysql.jdbc.Driver
   jdbc.user=root
   jdbc.password=root
   jdbc.characterEncoding=utf8
   

   ProvinceMapper.xml

   <?xml version="1.0" encoding="UTF-8" ?>
   <!DOCTYPE mapper
           PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
           "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
   
   <!-- 级联查询,一对多.例如:Province类中的一个属性 List<City> cityList ,对应City类的集合 -->
   <!-- namespace对应空Dao接口的全名 -->
   <mapper namespace="com.lanou3g.mybatis.dao.ProvinceDao">
       <resultMap id="province" type="com.lanou3g.mybatis.bean.Province">
           <id property="id" column="p_id" />
           <result property="pname" column="pname" />
           <collection property
   

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