如何在数据库中的相关表之间找到孤立条目？

Question

我有两个mySQL表，代表在特定期刊上发布的文档：

Documents: 

ID Title                          Journal
1  Monty: A New Study             6
2  New Discoveries about Python   17
3  An Unpublished Document        NULL 

Journals: 

ID Title 
6  Journal of Foo
10 Orphans Weekly 
17 Journal of Bar
99 Journal of Orphans

这里，文章“Monty：A New Study”发表在Journal of Foo上，“关于Python的新发现”发表在名为Journal of Bar的着名期刊上。

问题是，无论出于何种原因，都有没有相关文件的期刊标题，即＃10和＃99。我想删除没有相关文档的期刊的所有条目。我想做点什么：

delete from Journals where id is not one of (select journal from documents where journal is not null)  

但我是mySQL的新手并且坚持这个。

Answer 1

DELETE FROM `Journals` AS j WHERE j.ID NOT EXISTS(SELECT Journal FROM `Documents`);

...或

DELETE FROM `Journals` AS j WHERE j.ID NOT IN (SELECT Journal FROM `Documents`);

Answer 2

你可以这样做;

delete from Journals where id not in (SELECT Journal from Documents)