项目目录

一对一查询

一对一查询语句

一对一查询通常涉及通过外键关联两个表。例如，订单表和用户表的关系，可以通过订单的外键oid来查询对应的用户信息。

代码示例

OrderMapper接口

@Select("select * from orders")@Results({    @Result(id = true, column = "id", property = "id"),    @Result(column = "ordertime", property = "ordertime"),    @Result(column = "total", property = "total"),    @Result(column = "oid", property = "oid"),    @Result(property = "user", javaType = User.class, column = "oid",            one = @One(select = "cn.guardwhy.dao.UserMapper.findById", fetchType = FetchType.EAGER))})public interface OrderMapper {    void findAllWithUser();}

UserMapper接口

@Select("select * from user where id = #{id}")@Results({    @Result(column = "id", property = "id", id = true),    @Result(column = "user_name", property = "username"),    @Result(column = "birthday", property = "birthday"),    @Result(column = "sex", property = "sex"),    @Result(column = "address", property = "address")})public interface UserMapper {    User findById(Integer id);}

测试代码

// 查询一个订单，并同时查询该订单所属的用户@Testpublic void testOrderWithUser() {    SqlSession sqlSession = MybatisUtils.getSession();    OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);    List
   
     list = mapper.findAllWithUser();    for (Orders orders : list) {        System.out.println(orders);    }    sqlSession.close();}

一对多查询

一对多查询语句

一对多查询涉及通过一张表的主键到多张表的关联。例如，用户表和订单表的关系，可以通过用户表的id查询所有关联的订单信息。

代码示例

UserMapper接口

@Select("select * from user")@Results({    @Result(column = "id", property = "id", id = true),    @Result(column = "user_name", property = "username"),    @Result(column = "birthday", property = "birthday"),    @Result(column = "sex", property = "sex"),    @Result(column = "address", property = "address"),    @Result(property = "ordersList", javaType = List.class, column = "id",            many = @Many(select = "cn.guardwhy.dao.OrderMapper.findByOid"))})public interface UserMapper {    void findAllWithOrders();}

OrderMapper接口

@Select("select * from orders where id = #{oid}")public interface OrderMapper {    List
   
     findByOid(Integer oid);}

测试代码

// 查询一个用户，并同时查询该用户具有的订单@Testpublic void testUserWithOrders() {    SqlSession sqlSession = MybatisUtils.getSession();    UserMapper mapper = sqlSession.getMapper(UserMapper.class);    List
   
     userList = mapper.findAllWithOrders();    for (User user : userList) {        System.out.println(user);    }    sqlSession.close();}

多对多查询

多对多查询语句

多对多查询涉及通过中间表（关联表）查询两个表之间的关系。例如，用户表和角色表的关系，可以通过中间表user_role查询用户的角色信息。

代码示例

UserMapper接口

@Select("select * from user")@Results({    @Result(column = "id", property = "id", id = true),    @Result(column = "user_name", property = "username"),    @Result(column = "birthday", property = "birthday"),    @Result(column = "sex", property = "sex"),    @Result(column = "address", property = "address"),    @Result(property = "roleList", javaType = List.class, column = "id",            many = @Many(select = "cn.guardwhy.dao.RoleMapper.findByOid"))})public interface UserMapper {    void findAllWithRole();}

RoleMapper接口

@Select("select * from role r inner join user_role ur on r.id = ur.user_id where ur.role_id = #{id}")@Results({    @Result(id = true, column = "id", property = "id"),    @Result(column = "role_name", property = "roleName"),    @Result(column = "role_detail", property = "roleDetail")})public interface RoleMapper {    List
   
     findByOid(Integer id);}

测试代码

// 多对多查询测试@Testpublic void testUserWithRole() {    SqlSession sqlSession = MybatisUtils.getSession();    UserMapper mapper = sqlSession.getMapper(UserMapper.class);    List
   
     list = mapper.findAllWithRole();    for (User user : list) {        System.out.println(user);    }    sqlSession.close();}