6、MyBatis的各种查询功能

SelectMapper接口：

    //根据用户的id查询用户信息
    User getUserById(@Param("id") Integer Id);

SelectMapper.xml

    
    
        select * from t_user where id = #{id}
    
 
测试：
 
    @Test
    public void testGetUserById(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
        User userById = mapper.getUserById(1);
        System.out.println(userById);
    }
 
 
6.2、查询一个list集合 
SelectMapper接口
 
    //查询所有用户信息
    List getAllUser();
 
SelectMapper.xml
 
    
    
        select * from t_user
    

测试

    @Test
    public void testGetAllUser(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
//        for (User user : mapper.getAllUser()) {
//            System.out.println(user);
//        }
        List allUser = mapper.getAllUser();
        allUser.forEach(System.out::println);

    }

小结：

当查询的数据为多条时，不能使用实体类作为返回值，否则会抛出异常TooManyResultsException；但是若查询的数据只有一条，可以使用实体类或集合作为返回值。

SelectMapper接口

    //查询用户的总数量
    Integer getCount();

SelectMapper.xml

    
    
    
       select count(*) from t_user
    
 
测试
 
    @Test
    public void testGetCount(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
        Integer count = mapper.getCount();
        System.out.println(count);

    }
 
 
 
6.4、查询一条数据为map集合 
SelectMapper接口
 
  //查询用户（map）
    Map getUserByIdToMap(@Param("id") Integer id);
 
SelectMapper.xml
 
    
    
        select * from t_user where id = #{id}
    

测试

    @Test
    public void testGetUserByIdToMap(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
        Map userByIdToMap = mapper.getUserByIdToMap(1);
        //数据为null，将不会放在map集合中
        System.out.println(userByIdToMap);

    }

SelectMapper接口

//查询所有用户（map）
//    List> getAllUserToMap();

    @MapKey("id")
    Map getAllUserToMap();

SelectMapper.xml

    
    
        select * from t_user
    

测试

    @Test
    public void testGetAllUserToMap(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
//        for (Map stringObjectMap : mapper.getAllUserToMap()) {
//            System.out.println(stringObjectMap);
//        }

//        List> allUserToMap = mapper.getAllUserToMap();
//        allUserToMap.forEach(System.out::println);


        Map allUserToMap = mapper.getAllUserToMap();
        System.out.println(allUserToMap);
        //{1={password=123456, gender=男, id=1, age=23, email=123456@qq.com, username=admin},
        // 2={password=123, gender=男, id=2, age=23, email=123456@qq.com, username=root},
        // 4={password=111111, gender=女, id=4, age=33, email=1234@qq.com, username=root}}
    }