同一表上的比较选择（mysql）

我是mysql的新手，我想做一个简单的选择，但我想不出来

这就是我所拥有的：

I got this in a Table named Control:
| CODE | OFFICE |
|  1   |   usa  |
|  2   |   usa  |
|  3   |   usa  |
|  4   |   usa  |
|  5   |   usa  |
|  1   | china  |
|  3   | china  |
|  4   | china  |

And I need get this:

| CODE | OFFICE |
|  2   |   usa  |
|  5   |   usa  |

然后，选择代码，office，其中代码尚未在office=china注册

我必须做一个自连接或类似的东西，或者使用GROUP BY语句？

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我被困在这里。。。我真的非常感谢您的帮助。

我想这很有效

 create table Test(id integer, code integer, office varchar(100));

insert into Test(id, code, office) values(1, 1, "usa"),(2, 2, "usa"),(3, 3, "usa"),(4, 4, "usa"),(5, 5, "usa"),(6, 1, "china"),(7, 3, "china"),(8, 4, "china");

select * from Test;

Select * from Test where code NOT IN (select code from Test where office =   "china");

您必须使用子查询

> P>你可以在<代码>代码<代码>上做一个“自左连接”，只考虑右边没有匹配的行，即右侧值<代码>为null

SELECT 
  tleft.*
FROM Control AS tleft
LEFT JOIN Control AS tright
  ON tright.Code = tleft.Code AND 
     tright.Office = 'china' 
WHERE tleft.Office = 'usa' AND 
      tright.Code IS NULL -- this filters no matching code found in china