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# Check Table For Any Oprhaned 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.