sql ruby-on-rails postgresql aggregate-functions aggregate-filter

Rails Query - Group By with 2 groups

In my website my users have an attribute localidade. This specifies where they live.

I'm trying to do a query where I group the results the following way:

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

I want the number of users from New York and the number of users from anywhere else but New York.

I tried this:

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

but since I don't have any users from new york and only 1 not from New York it returns:

{false => 1}

Am I able to give aliases to groups? Is there any way of grouping this way the results?

I'm gonna use the results for a Pie Graph from Graphkick.

Solution

You could write your query :

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")

Since 9.4, you can use FILTER with aggregate expression :

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")

I created a Table to explain and test the above sql, and they worked as expected :

[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)

Let me now, jump to the rails console, and test the equivalent Rails code :

[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}]

Update

You can remove those nil from DB level only :

Rails code :

[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}]

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)

Update- II

The classical approach to get the output same as FILTER :

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=#

And In Rails way :-

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)>