我正在尝试选择具有MAX微博计数的用户:
SELECT "name", count(*) FROM "users"
INNER JOIN "microposts" ON "microposts"."user_id" = "users"."id"
GROUP BY users.id
这回来了
"Delphia Gleichner";15
"Louvenia Bednar IV";10
"Example User";53
"Guadalupe Volkman";20
"Isabella Harvey";30
"Madeline Franecki II";40
但是我想只选择"Example User";53(具有MAX微博计数的用户)
我试图添加,HAVING MAX(count*)但这不起作用.
我的应用程序中有祖先树.
我想仅在同一级别上实现记录的唯一性.
让我解释
记录:
#<Folder id: 1, title: "folder1", ... ancestry: nil>,
#<Folder id: 2, title: "folder2", ... ancestry: "1">,
#<Folder id: 3, title: "folder3", ... ancestry: "1/2">
如果我想创建folder (:title => 'folder2', :ancestry => '1')它应该引发错误title is olready taken,
但如果我创建 folder (:title => 'folder2', :ancestry => nil or "1/2")它应该创建新的记录.
可能有人有这个问题,可以给出建议或答案
(在祖先方法中@ elem.siblings返回@elem级别的所有记录)