Compare a list in C# to a table in a database

Question

I need to compare a list of sequential integers created from a C# Windows Form to a database table of integers, indicating if there are duplicates.

I have a version that works, below, but I'm assuming it's probably the least efficient way of doing this - comparing the C# list one by one to each integer in the database table.

Should I get the integers from the database into C#, and then compare? Or is there a sql way of asking:

if any item in list A is contained in list B, etc.... without comparing each number one-by-one?

I've seen lots of opinions on just looking for 1 item in a database, but I need an efficient way of comparing C# lists of sometimes 5,000 or more to a database table that can end up having hundreds of thousands of records.

public static string VerifyManufacturingSerialOnly(int count, int beginning)
        {
            string duplicateSerials = "";
            int currentSerial = beginning;

            for (int i = 0; i < count; i++)
            {
                OleDbConnection connection = BadgeDatabaseDB.GetConnection();
                string checkStatement
                    = "SELECT * "
                    + "FROM SerialNumbersMFG "
                    + "WHERE SerialNumber = @CurrentSerial";
                OleDbCommand command =
                    new OleDbCommand(checkStatement, connection);
                command.Parameters.AddWithValue("@CurrentSerial", currentSerial);                

                try
                {
                    connection.Open();
                    OleDbDataReader dataReader =
                        command.ExecuteReader(CommandBehavior.SingleRow);
                    if (dataReader.Read())
                    {
                        duplicateSerials +=
                            "Serial # " +
                            currentSerial +
                            " already exists in order # " +
                            dataReader["OrderNumber"].ToString() + "\n";
                    }
                    else {  }
                }
                catch (OleDbException ex)
                {
                    throw ex;
                }
                finally
                {
                    connection.Close();
                }
                currentSerial++;
                i++;
            }
            return duplicateSerials;

Answer 1

Two ways:

1. Get all the id's from the database SELECT SerialNumber FROM SerialNumbersMFG and then you can use linq list1.Intersect(list2)
2. As posted in the comment, most DB have the IN clause, so you can use the query like: SELECTSerialNumber FROM SerialNumbersMFG WHERE SerialNumber IN (1,2,3...) which will also do the required thing.

Since you mention that the DB can have hundreds of thousand of record I would suggest that second way is better. Please use stringbuilder for concatenation.