Mybatis的各种查询功能

 1.若查询出来的数据只有一条

                a.可以通过实体类对象接受
                d.也可以通过list集合来进行接受
                c.可以通过map集合接收

（ 结果：{update_time=2022-07-17T12:15:23, deleted=0, create_time=2022-07-16T22:06:50, name=双双4, id=1548240344215633922, version=3, age=23, email=146578237@qq.com}
以字段为键以字段所对应的值为值 )

    /*
    * 查询单条数据
    * */
    Users selectUsers(@Param("name") String name);
 
 
 "selectUsers" resultType="users">
        select * from users where name = #{name}
 
 
 
 
  @Test
    public void  testSelectMapper(){
        SqlSession  sqLsession = SQLsession.getSqlsession();
        SelectMapper mapper = sqLsession.getMapper(SelectMapper.class);
        Users users = mapper.selectUsers("双双");
        System.out.println(users);
    }

2.若查询出来的数据是多条
                a.可以通过list集合来进行接收
                b.可以通过map类型的list进行接收
                c.可以在mapper接口的方法上添加上@MapKey注解，此时就可以将每一条数据转换的map的集合作为值，以某一个字段的值为键，放入同一个map集合中

(注意在多条数据的时候一定不能用实体类对象来进行接收，如果用实体类对象来接收的话会抛出异常TooManyResultsException)

/*
     * 查询多条数据
     * */
    List selectUser(@Param("name") String name);
 
 
 "selectUser" resultType="users">
        select * from users where name = #{name}
 
 
 
 
@Test
    public void  testSelectMappers(){
        SqlSession  sqLsession = SQLsession.getSqlsession();
        SelectMapper mapper = sqLsession.getMapper(SelectMapper.class);
        List users = mapper.selectUser("双双");
        System.out.println(users);
    }

3、查询数据的总记录数

(注意：因为返回值为Integer，所以在mapper.xml映射文件当中resutlType应该返回Integer
还可以写成 int , Integer, integer 因为这是mybatis默认规定:详情可查阅mybatis官网)

/*
    * 查询总记录数
    * */
    Integer getCount();
 
"getCount" resultType="Integer">
        select count(*) from users
 
 
 
    @Test
    public void  tsetgetCount(){
        SqlSession  sqLsession = SQLsession.getSqlsession();
        SelectMapper mapper = sqLsession.getMapper(SelectMapper.class);
        int users = mapper.getCount();
        System.out.println(users);
    }

4、查询一条数据为map集合

/*
    * 根据name查询用户信息为一个map集合
    * */
    Map getUserBynameToMap(@Param("name") String name);
 
 
"getUserBynameToMap" resultType="Map">
        select * from users where name = #{name}
 
 
 
@Test
    public void  tsetgetUserBynameToMap(){
        SqlSession  sqLsession = SQLsession.getSqlsession();
        SelectMapper mapper = sqLsession.getMapper(SelectMapper.class);
        Map users = mapper.getUserBynameToMap("双双4");
        System.out.println(users);
    }

5、查询多条数据为map集合

 /*
    * 查询所有的用户信息为map集合
    * */
//    List> getAllUserToMap();
@MapKey("id")
    Map getAllUserToMap();
 
 
 
"getAllUserToMap" resultType="Map">
        select * from users
 
 
 
@Test
    public void  tsetgetAllUserToMap(){
        SqlSession  sqLsession = SQLsession.getSqlsession();
        SelectMapper mapper = sqLsession.getMapper(SelectMapper.class);
//        List> users = mapper.getAllUserToMap();
        Map users = mapper.getAllUserToMap();
        System.out.println(users);
    }