实现复杂关系映射之前我们可以在映射文件中通过配置来实现，使用注解开发后，我们可以使用@Results注解，@Result注解，@One注解，@Many注解组合完成复杂关系的配置

多表查询：一对一查询

用户表和订单表的关系为，一个用户有多个订单，一个订单只从属于一个用户

对应的sql语句：
select * from orders;
select * from user where id=查询出订单的uid;

主要注解：
@Result(column = "id",property = "id")
one = @One(select = "com.study.mapper.UserMapper.findById")

步骤1：创建订单类

public class Order {
    private int id;
    private Date ordertime;
    private double total;
    //当前订单属于哪一个用户
    private User user;

步骤2：创建OrderMapper接口

第一种方式：

public interface OrderMapper {
    @Select("select *,o.id oid from orders o,user u where o.uid=u.id")
    @Results({
            @Result(column = "oid",property = "id"),
            @Result(column = "ordertime",property = "ordertime"),
            @Result(column = "total",property = "total"),
            @Result(column = "uid",property = "user.id"),
            @Result(column = "username",property = "user.username"),
            @Result(column = "password",property = "user.password")
    })
    public List<Order> findAll();
}

第二种方式：

public interface OrderMapper {
    @Select("select * from orders")
    @Results({
            @Result(column = "id",property = "id"),
            @Result(column = "ordertime",property = "ordertime"),
            @Result(column = "total",property = "total"),
            @Result(
                    property = "user", //要封装的属性名称
                    column = "uid", //根据那个字段去查询user表的数据
                    javaType = User.class, //要封装的实体类型
                    //select属性 代表查询那个接口的方法获得数据   UserMapper的findById方法
                    one = @One(select = "com.study.mapper.UserMapper.findById")
            )
    })
    public List<Order> findAll();

}

步骤3：配置核心配置文件

  <!--加载映射关系-->
    <mappers>
        <!--指定接口所在的包-->
        <package name="com.study.mapper"></package>
    </mappers>

步骤4：编写测试类

public class MyBatisTest2 {
    private OrderMapper mapper;
    @Before
    public void before() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        mapper = sqlSession.getMapper(OrderMapper.class);
    }

    @Test
    public void testSave(){
        List<Order> all = mapper.findAll();
        for (Order order : all) {
            System.out.println(order);
        }
    }
}

多表查询：一对多查询

1个用户对应多个订单

对应的sql语句：
select * from user;
select * from orders where uid=查询出用户的id;

使用主要注解：
@Many
@Result

步骤1：创建User类

public class User {

    private int id;
    private String username;
    private String password;
    private Date birthday;
    //当前用户具有的订单
    private List<Order> orderList;
}

步骤2： 修改OrderMapper接口：

 @Select("select * from orders where uid=#{uid}")
  public List<Order> findByUid(int uid);

步骤3：修改Usermapper接口

@Select("select * from user")
    @Results({
<!--id=true代表标识为一个id-->
            @Result(id=true ,column = "id",property = "id"),
            @Result(column = "username",property = "username"),
            @Result(column = "password",property = "password"),
          <!--orderList与 User类里面的保持一致-->
            @Result(
                    property = "orderList",
                    column = "id",
                    javaType = List.class,
                    many = @Many(select = "com.study.mapper.OrderMapper.findByUid")
            )
    })
    public List<User> findUserAndOrderAll();

步骤4 核心配置文件导入mapper接口所在的包：

  <!--加载映射关系-->
    <mappers>
        <!--指定接口所在的包-->
        <package name="com.study.mapper"></package>
    </mappers>

步骤5 测试：

public class MyBatisTest3 {
    private UserMapper mapper;
    @Before
    public void before() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        mapper = sqlSession.getMapper(UserMapper.class);
    }
    @Test
    public void testSave(){
        List<User> userAndOrderAll = mapper.findUserAndOrderAll();
        for (User user : userAndOrderAll) {
            System.out.println(user);
        }
    }
}

多对多查询

用户表和角色表的关系为，一个用户有多个角色，一个角色被多个用户使用
多对多查询的需求：查询用户同时查询出该用户的所有角色

对应的sql语句：
select * from user;
select * from role r,user_role ur where r.id=ur.role_id and ur.user_id=用户的id

步骤1：创建Role实体，修改User实体

public class User {
    private int id;
    private String username;
    private String password;
    private Date birthday;
    //代表当前用户具备哪些订单
    private List<Order> orderList;
    //代表当前用户具备哪些角色
    private List<Role> roleList;
}

public class Role {
    private int id;
    private String rolename;

}

步骤2：添加UserMapper接口方法

List<User> findAllUserAndRole();

步骤3：使用注解配置Mapper

public interface UserMapper {
    @Select("select * from user")
    @Results({
        @Result(id = true,property = "id",column = "id"),
        @Result(property = "username",column = "username"),
        @Result(property = "password",column = "password"),
        @Result(property = "birthday",column = "birthday"),
        @Result(property = "roleList",column = "id",
                javaType = List.class,
                many = @Many(select = "com.study.mapper.RoleMapper.findByUid"))
})
List<User> findAllUserAndRole();}

public interface RoleMapper {
    @Select("select * from role r,user_role ur where r.id=ur.role_id and ur.user_id=#{uid}")
    List<Role> findByUid(int uid);
}

步骤4：测试结果

public class MyBatisTest4 {
    private UserMapper mapper;
    @Before
    public void before() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        mapper = sqlSession.getMapper(UserMapper.class);
    }


    @Test
    public void testSave(){
        List<User> userAndRoleAll = mapper.findUserAndRoleAll();
        for (User user : userAndRoleAll) {
            System.out.println(user);
        }
    }
}