Question

在我的网站中，我的用户有一个属性localidade。这指定了他们居住的地方。

我尝试进行查询，我按以下方式对结果进行分组：

localidade   | Number of Users
-------------+--------------
New York     | 6
Not New York | 8

我想要来自纽约的用户数量以及除纽约之外的其他任何地方的用户数量。

我试过了：

User.group("lower(localidade) = 'new york'").count

但是因为我没有来自纽约的任何用户，只有1个不是来自纽约，所以它返回：

{false => 1}

我可以给组别名吗？有没有办法将结果分组？

我将把结果用于Graphkick的Pie Graph。

Answer 1

您可以编写查询：

User.group("lower(localidade)")
    .select("CASE WHEN lower(localidade) = 'new york' THEN COUNT(id) END AS NewYork, 
             CASE WHEN lower(localidade) != 'new york' THEN COUNT(id) END AS Non-NewYork")

由于 9.4 ，您可以将FILTER与汇总表达式一起使用：

User.group("lower(localidade)")
    .select("COUNT(id) FILTER (WHERE lower(localidade) != 'new york') AS NonNewyork, 
             COUNT(id) FILTER (WHERE lower(localidade) = 'new york') AS Newyork")

我创建了一个Table来解释和测试上面的sql，它们按预期工作：

[shreyas@rails_app_test (master)]$ rails db
psql (9.4.1)
Type "help" for help.

app_development=# select id, location, name from people;
 id | location | name
----+----------+------
  2 | X        | foo
  3 | X        | foo
  4 | Y        | foo
(3 rows)

app_development=# SELECT COUNT(id) FILTER(WHERE lower(location) != 'x') AS Non_X_loc, COUNT(id) FILTER (WHERE lower(location) = 'x') AS X_loc FROM "people";
 non_x_loc | x_loc
-----------+-------
         1 |     2
(1 row)

现在让我跳转到rails控制台，测试等效的Rails代码：

[2] pry(main)> p = Person.select("COUNT(id) FILTER(WHERE lower(location) != 'x') AS Non_X_loc, COUNT(id) FILTER (WHERE lower(location) = 'x') AS X_loc ")
  Person Load (0.5ms)  SELECT COUNT(id) FILTER(WHERE lower(location) != 'x') AS Non_X_loc, COUNT(id) FILTER (WHERE lower(location) = 'x') AS X_loc  FROM "people"
=> [#<Person:0x007fd85ed71980 id: nil>]
[3] pry(main)> p.first.attributes
=> {"id"=>nil, "non_x_loc"=>1, "x_loc"=>2}
[6] pry(main)> Person.group("lower(location)").select("CASE WHEN lower(location) = 'x' THEN COUNT(id) END AS X_loc, CASE WHEN lower(location) != 'x' THEN COUNT(id) END AS Non_X_loc")
  Person Load (0.6ms)  SELECT CASE WHEN lower(location) = 'x' THEN COUNT(id) END AS X_loc, CASE WHEN lower(location) != 'x' THEN COUNT(id) END AS Non_X_loc FROM "people" GROUP BY lower(location)
=> [#<Person:0x007fd8608281e8 id: nil>, #<Person:0x007fd860828008 id: nil>]
[7] pry(main)> p = _
=> [#<Person:0x007fd8608281e8 id: nil>, #<Person:0x007fd860828008 id: nil>]
[8] pry(main)> p.map { |rec| rec.attributes }
=> [{"id"=>nil, "x_loc"=>nil, "non_x_loc"=>1}, {"id"=>nil, "x_loc"=>2, "non_x_loc"=>nil}]
[9] pry(main)> p.map { |rec| rec.attributes.except('id') }
=> [{"x_loc"=>nil, "non_x_loc"=>1}, {"x_loc"=>2, "non_x_loc"=>nil}]

<强>更新

您只能从数据库级别删除nil：

Rails代码：

[shreyas@rails_app_test (master)]$ rails c
Loading development environment (Rails 4.2.0)
[1] pry(main)> Person.group("lower(location)").select("CASE WHEN lower(location) = 'x' THEN COUNT(id) ELSE 0 END AS X_loc, CASE WHEN lower(location) != 'x' THEN COUNT(id) ELSE 0 END AS Non_X_loc")
  Person Load (0.9ms)  SELECT CASE WHEN lower(location) = 'x' THEN COUNT(id) ELSE 0 END AS X_loc, CASE WHEN lower(location) != 'x' THEN COUNT(id) ELSE 0 END AS Non_X_loc FROM "people" GROUP BY lower(location)
=> [#<Person:0x007fd858c100b0 id: nil>, #<Person:0x007fd860853e88 id: nil>]
[2] pry(main)> p = _
=> [#<Person:0x007fd858c100b0 id: nil>, #<Person:0x007fd860853e88 id: nil>]
[3] pry(main)> p.map { |rec| rec.attributes }
=> [{"id"=>nil, "x_loc"=>0, "non_x_loc"=>1}, {"id"=>nil, "x_loc"=>2, "non_x_loc"=>0}]
[4] pry(main)> p.map { |rec| rec.attributes.except('id') }
=> [{"x_loc"=>0, "non_x_loc"=>1}, {"x_loc"=>2, "non_x_loc"=>0}]
[5] pry(main)> p = Person.select("count(CASE WHEN lower(location) = 'x' THEN 1 END) AS X_loc, count(CASE WHEN lower(location) != 'x' THEN 1 END) AS Non_X_loc").group("lower(location)")
  Person Load (0.9ms)  SELECT count(CASE WHEN lower(location) = 'x' THEN 1 END) AS X_loc, count(CASE WHEN lower(location) != 'x' THEN 1 END) AS Non_X_loc FROM "people" GROUP BY lower(location)
=> [#<Person:0x007fd85b150f78 id: nil>, #<Person:0x007fd85b150230 id: nil>]
[6] pry(main)> p.map { |rec| rec.attributes.except('id') }
=> [{"x_loc"=>0, "non_x_loc"=>1}, {"x_loc"=>2, "non_x_loc"=>0}]

<强> SQL

app_development=# select CASE WHEN lower(location) = 'x' THEN COUNT(id) ELSE 0 END AS X_loc, CASE WHEN lower(location) != 'x' THEN COUNT(id) ELSE 0 END AS Non_X_loc from people group by lower(location);
 x_loc | non_x_loc
-------+-----------
     0 |         1
     2 |         0
(2 rows)
app_development=# select count(CASE WHEN lower(location) = 'x' THEN 1 END) AS X_loc, count(CASE WHEN lower(location) != 'x' THEN 1 END) AS Non_X_loc from people group by lower(location);
 x_loc | non_x_loc
-------+-----------
     0 |         1
     2 |         0
(2 rows)

更新 - II

将输出与 FILTER 相同的经典方法：

app_development=# select count(CASE WHEN lower(location) = 'x' THEN 1 END) AS X_loc, sum(CASE WHEN lower(location) != 'x' THEN 1 END) AS Non_X_loc from people;
 x_loc | non_x_loc
-------+-----------
     2 |         1
(1 row)

app_development=# select sum(CASE WHEN lower(location) = 'x' THEN 1 END) AS X_loc, sum(CASE WHEN lower(location) != 'x' THEN 1 END) AS Non_X_loc from people;
 x_loc | non_x_loc
-------+-----------
     2 |         1
(1 row)

app_development=# select id, location, name from people;
 id | location | name
----+----------+------
  2 | X        | foo
  3 | X        | foo
  4 | Y        | foo
(3 rows)

app_development=#

以Rails的方式： -

Loading development environment (Rails 4.2.0)
[1] pry(main)> p = Person.select("sum(CASE WHEN lower(location) = 'x' THEN 1 END) AS X_loc, sum(CASE WHEN lower(location) != 'x' THEN 1 END) AS Non_X_loc")
  Person Load (0.6ms)  SELECT sum(CASE WHEN lower(location) = 'x' THEN 1 END) AS X_loc, sum(CASE WHEN lower(location) != 'x' THEN 1 END) AS Non_X_loc FROM "people"
=> [#<Person:0x007fd85b6e6a78 id: nil>]
[2] pry(main)> p.first.attributes.except("id")
=> {"x_loc"=>2, "non_x_loc"=>1}
[3] pry(main)> p = Person.select("count(CASE WHEN lower(location) = 'x' THEN 1 END) AS X_loc, count(CASE WHEN lower(location) != 'x' THEN 1 END) AS Non_X_loc")
  Person Load (0.5ms)  SELECT count(CASE WHEN lower(location) = 'x' THEN 1 END) AS X_loc, count(CASE WHEN lower(location) != 'x' THEN 1 END) AS Non_X_loc FROM "people"
=> [#<Person:0x007fd85b77f098 id: nil>]
[4] pry(main)> p.first.attributes.except("id")
=> {"x_loc"=>2, "non_x_loc"=>1}
[5] pry(main)>

Answer 2

老实说，你的工作正常，你只需要了解如果true（或false的哈希值没有值，那么该值必须默认为零，您可以使用.to_i对nil值进行操作。所以，例如：

ny_count = User.group("lower(localidade) = 'new york'").count

"New York: #{ny_count[true].to_i}
Not New York: #{ny_count[false].to_i}
"

Rails查询 - 分组包含2个组

2 个答案: