sql - GROUP BY 和 COUNT 使用 ActiveRecord

Question

引用这个: Is there any difference between GROUP BY and DISTINCT

Given a table that looks like this:

name
------
barry
dave
bill
dave
dave
barry
john
This query:

SELECT name, count(*) AS count FROM table GROUP BY name;
Will produce output like this:

name    count
-------------
barry   2
dave    3
bill    1
john    1

对于 ActiveModel 使用 COUNT 执行 GROUP BY 的正确 Rails 约定是什么？

Answer 1

Distinct 和 Group By 会给你不同的结果。要获得您期望的结果，您需要使用

Person.group(:name).count
(1.2ms)  SELECT COUNT(*) AS count_all, name AS name FROM "people" GROUP BY "people"."name"
=> {"Dan"=>3, "Dave"=>2, "Vic"=>1} 

如上所示，组将以散列形式返回内容。虽然 distinct 只返回总人数，如下所示。

Person.distinct(:name).count
(0.4ms)  SELECT DISTINCT COUNT(DISTINCT "people"."id") FROM "people"
=> 6