查询联接获取，将最新创建的字段添加到行

Question

查询很困难。

我有一个名为project_slugs的表，其中包含以下字段id, project_id, slug, created。

我有一个主表projects，其中包含各个字段。

通过projects.id和project_slugs.project_id存在外键关系。

project_slugs可以为任何给定项目包含多个子段。

我想获得一个单一的结果，其中包含projects表中的所有字段以及该项目的最近创建的 projects_slug.slug。这可以通过WHERE projects_slug.slug = 'some-slug'完成，其中“ some-slug”可能是也可能不是最近的。

我能够成功加入表格，但是我不确定如何合并上面的粗体逻辑。

这是我当前的查询：

SELECT projects.*, project_slugs.slug 
FROM `project_slugs` LEFT JOIN 
     `projects`
     ON project_slugs.project_id = projects.id
WHERE project_slugs.slug = 'some-slug'

插头：

项目：

预期的输入输出：

SELECT projects.*, project_slugs.slug
FROM `project_slugs` LEFT JOIN 
     `projects`
     ON project_slugs.project_id = projects.id
WHERE project_slugs.slug = 'star-management-week-2015'

期望的输出将是您在最后一张图像中看到的结果+由slug确定的最新的 project_slugs.created值。给定 any project_slugs.slug（新的或旧的）作为标识符。我什至不确定在sql中是否可行。逻辑在php中非常简单。

Answer 1

您还应该将联接与子查询一起用于由project_id创建的最大组

SELECT projects.*, 
project_slugs.slug 
FROM projects 
inner join  (
  select project_id, max(created) max_created 
  from  project_slugs 
  group by  project_id 
) t on t.project_id = projects.id 
inner join  `project_slugs` ON project_slugs.project_id = projects.id 
AND project_slugs.created = t.max_created 

Answer 2

我想您想要任何有问题的项目中的最新项目。如果是这样：

 NSArray *services = @[[CBUUID UUIDWithString:@"180A"]];
_connectedPeripherals = [_btManager retrieveConnectedPeripheralsWithServices:services];

如果您实际上只希望最近的子段是指定的子段的项目，则在MySQL中，可以使用扩展的select p.*, (select ps.slug from project_slugs ps where ps.project_id = p.id order by ps.created desc limit 1 ) as most_recent_slug from projects p where exists (select 1 from project_slugs ps where ps.project_id = p.id and ps.slug = 'some-slug' ); 子句。将having替换为：

where