Add Check Table For Any Orphaned Records as a postgres til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_852 TILs and counting..._
+_853 TILs and counting..._
 
 ---
 
@@ -391,6 +391,7 @@ _852 TILs and counting..._
 - [Between Symmetric](postgres/between-symmetric.md)
 - [Capitalize All The Words](postgres/capitalize-all-the-words.md)
 - [Change The Current Directory For psql](postgres/change-the-current-directory-for-psql.md)
+- [Check Table For Any Oprhaned Records](postgres/check-table-for-any-orphaned-records.md)
 - [Checking Inequality](postgres/checking-inequality.md)
 - [Checking The Type Of A Value](postgres/checking-the-type-of-a-value.md)
 - [Clear The Screen In psql](postgres/clear-the-screen-in-psql.md)

postgres/check-table-for-any-orphaned-records.md

@@ -0,0 +1,27 @@
+# Check Table For Any Orphaned Records
+
+If you don't have a foreign key constraint in place to enforce the relationship
+between records in two different tables, then there are a number of ways you
+could end up with orphaned records. Orphaned records are records that have a
+value in an `*_id` column when that value doesn't correspond to any record in
+the related table.
+
+For example, let's say we have an `authors` table with an `id` column and a
+`books` table with an `author_id` column. If there is a book record with an
+`author_id` value that doesn't resolve to any record in the `authors` table,
+then that book is an orphaned record.
+
+You can find out if a table has orphaned records like so:
+
+```sql
+select count(*)
+  from books
+  left join authors
+    on books.author_id = authors.id
+  where authors.id is null
+    and books.author_id is not null;
+```
+
+We select from our table with the foreign key (`books`) and _left join_ it
+against the related table (`authors`). If there are any book records where the
+joined author row is `null`, then that book is orphaned.