Add Select Values For SQL Counts as a rails til.

rails/select-value-for-sql-counts.md

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+# Select Value For SQL Counts
+
+If you are like me and prefer writing raw SQL over the Arel DSL for counting
+stuff in your database, then the `select_value` method will come in handy.
+Write a command similar to the following with a type cast to get the count
+of _whatever_.
+
+```ruby
+> sql = 'select count(*) from posts where published_at is not null'
+=> "select count(*) from posts where published_at is not null"
+> ActiveRecord::Base.connection.select_value(sql).to_i
+   (0.6ms)  select count(*) from posts where published_at is not null
+=> 42
+```