Question

参考： 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

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

Answer 1

Distinct和Group By会给您不同的结果。要获得您希望使用的结果

Person.all.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}

如上所述，group将返回哈希值。虽然不同只是返回总人数，见下文。

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

Answer 2

请注意，接受的答案将返回哈希：

Tagging.joins(:tag).group(:name).size
   (0.4ms)  SELECT COUNT(*) AS count_all, `name` AS name FROM `taggings` INNER JOIN `tags` ON `tags`.`id` = `taggings`.`tag_id` GROUP BY `name`
 => {"applesauce"=>1, "oranges"=>2}

但是，如果要返回计数以及联接中来自不同表的一些列，该怎么办。然后，您还需要使用选择的ActiveRecord查询：

collection = Tagging.select('COUNT(*) as total, taggings.taggable_id, taggings.taggable_type').joins(:tag).where(taggable_type: 'LineItem', taggable_id: ['5cad0dcc3ed1496086000031', '5cad0dcd3ed1496086000081'] ).group(:name)

collection.each do |item|
  puts item.taggable_id
  puts item.total
end

5cad0dcc3ed1496086000031
1
5cad0dcc3ed1496086000031
2

使用第二种方法，您可以获取有关联接关系的其他详细信息，而无需任何其他查询或循环构造。

Answer 3

另一个选择：

Person.select(:name, 'COUNT(name)').group(:name)

这会产生一系列具有属性计数的人

GROUP BY和COUNT使用ActiveRecord

3 个答案: