Readarr/src/NzbDrone.Core/Housekeeping/Housekeepers/CleanupOrphanedBooks.cs

using Dapper;
using NzbDrone.Core.Datastore;

namespace NzbDrone.Core.Housekeeping.Housekeepers
{
    public class CleanupOrphanedBooks : IHousekeepingTask
    {
        private readonly IMainDatabase _database;

        public CleanupOrphanedBooks(IMainDatabase database)
        {
            _database = database;
        }

        public void Clean()
        {
            using (var mapper = _database.OpenConnection())
            {
                mapper.Execute(@"DELETE FROM ""Books""
                                     WHERE ""Id"" IN (
                                     SELECT ""Books"".""Id"" FROM ""Books""
                                     LEFT OUTER JOIN ""Authors""
                                     ON ""Books"".""AuthorMetadataId"" = ""Authors"".""AuthorMetadataId""
                                     WHERE ""Authors"".""Id"" IS NULL)");
            }
        }
    }
}