mysql - 如何找到数据库中相关表之间的孤立条目？

Question

我有两个 mySQL 表，代表在特定期刊上发表的文档:

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3  An Unpublished Document        NULL 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，文档“Monty: A New Study”发表在 Journal of Foo 上，“New Discoveries about Python”发表在著名期刊 Journal of Bar 上。

问题是，无论出于何种原因，有些期刊标题没有关联文档，即#10 和#99。我想删除所有没有关联文档的期刊条目。我想做一些类似的事情:

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

但是我是 mySQL 的新手，并且陷入了困境。

Answer 1

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

或者...

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