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

README.md

@@ -9,7 +9,7 @@ and pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_924 TILs and counting..._
+_925 TILs and counting..._
 
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@@ -582,6 +582,7 @@ _924 TILs and counting..._
 - [Perform SQL Explain With ActiveRecord](rails/perform-sql-explain-with-activerecord.md)
 - [Polymorphic Path Helpers](rails/polymorphic-path-helpers.md)
 - [Pretend Generations](rails/pretend-generations.md)
+- [Query A Single Value From The Database](rails/query-a-single-value-from-the-database.md)
 - [Read-Only Models](rails/read-only-models.md)
 - [Remove The Default Value On A Column](rails/remove-the-default-value-on-a-column.md)
 - [Rescue From](rails/rescue-from.md)

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

@@ -0,0 +1,22 @@
+# 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.