Add Query A Single Value From The Database as a rails til

rails/query-a-single-value-from-the-database.md

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+# Query A Single Value From The Database
+
+In a Rails context, most database interactions tend to happen through the ORM
+(e.g. `Book.find("123")`). There is a general purpose escape hatch that lets
+you execute a SQL statement directly against the DB -- `execute`. The resulting
+value of `execute`, however, tends to be a little clunky to work with.
+
+If you just need a single value from the DB, use the
+[`select_value`](https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/DatabaseStatements.html#method-i-select_value)
+method.
+
+```ruby
+> statement = "select gen_random_uuid()"
+> ActiveRecord::Base.connection.select_value(statement)
+   (5.0ms)  select gen_random_uuid()
+ => "abc2e780-f442-418b-afa3-56f0ccd0a903"
+```
+
+This is the cleanest way to get the result of a "single value" query.
+
+If you happen to pass in a query that results in more than one row or column,
+it will return the value of the first column from the first row.