Add Check Table For Any Orphaned Records as a postgres til

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

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+# 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.