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:

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.