How to bulk update 1000 records using C#/SQL

Question

I am reading from a table with a million records, I perform some modifications to some columns and I would like to save them back in bulk. I would like to perform a bulk update after every 10000 records.

I'm using .Net Core 3.1

My code :

public void FixText()
        {
            SqlConnection cnn = new SqlConnection(strConn);
            string queryString = "SELECT ClientID, ServerName, Text FROM  [dbo].[Client]";
            SqlCommand selectCmd = new SqlCommand(queryString, cnn);
            cnn.Open();
            int j = 0;
            SqlDataReader reader = selectCmd.ExecuteReader();
            List<Client> clients = new List<Client>();
            try
            {
                while (reader.Read())
                {
                    j++;
                    ClientID = (int)reader["ClientID"];
                    ServerName = reader["ServerName"].ToString();
                    Text = reader["Text"].ToString();
                    
                    
                    //Modify Text & set ServerName
                    string Text = UpdateText(Text);

                    if text.StartWith("doo"){ServerName = "re";}

                    //perform bulk update
                    if (j > 10000)
                    {
                        Client client = new Client()
                        {
                            ClientID = (int)ClientID,
                            ServerName = ServerName,
                            Text = Text,
                        };
                        clients.Add(client);
                        //I'm struggling here on what to use to do a bulk update
                       
                        j = 0;
                    }
                }

            }
            catch (Exception ex)
            {
                throw ex;
            }
            finally
            {
                reader.Close();
            }
            cnn.Close();
        }

Any Help is appreciated!

Answer 1

You have two options, either use MERGE statement, or UPDATE.

I will do UPDATE option, as it's the easiest one. (This would need FastMember nuget).

private void ExecuteSql(SqlConnection connection , string sql , SqlParameter[] parameters = null)
{
    if(connection == null)
    {
        throw new ArgumentNullException(nameof(connection));
    }

    if(string.IsNullOrWhiteSpace(sql))
    {
        throw new ArgumentNullException(nameof(sql));
    }
    
    using(var command = new SqlCommand(sql , connection))
    {
        if(parameters?.Length > 0)
        {
            command.Parameters.AddRange(parameters);
        }

        if(connection.State != ConnectionState.Open)
            connection.Open();

        command.ExecuteNonQuery();
    }
}

private void ExecuteBulkCopy<T>(SqlConnection connection , IEnumerable<T> entries , string destinationTableName , string[] columns = null , int batchSize = 1000000)
{
    if(connection == null)
    {
        throw new ArgumentNullException(nameof(connection));
    }

    if(entries == null)
    {
        throw new ArgumentNullException(nameof(entries));
    }

    if(string.IsNullOrWhiteSpace(destinationTableName))
    {
        throw new ArgumentNullException(nameof(destinationTableName));
    }

    if(connection.State != ConnectionState.Open)
        connection.Open();

    using(SqlBulkCopy sbc = new SqlBulkCopy(connection)
    {
        BulkCopyTimeout = 0 ,
        DestinationTableName = destinationTableName ,
        BatchSize = batchSize
    })
    {
        using(var reader = ObjectReader.Create(entries , columns))
        {
            sbc.WriteToServer(reader);
        }
    }
}

private IEnumerable<Client> GetUpdatedClients(SqlConnection connection)
{
    using(var command = new SqlCommand("SELECT ClientID, ServerName, Text FROM  [dbo].[Client]", connection))
    {
        connection.Open();
        using(SqlDataReader reader = _connection.ExecuteReader(query , parameters))
        {
            if(reader.HasRows)
            {
                while(reader.Read())
                {
                    if(reader.IsDBNull(x)) { continue; }

                    var clientId   = (int)reader["ClientID"]; 
                    var serverName = reader["ServerName"]?.ToString(); 
                    var text = reader["Text"]?.ToString();
                    
                    //Modify Text & set ServerName
                    string textUpdated = UpdateText(text);                  
                    
                    if(textUpdated.StartWith("doo"))
                    {
                        serverName = "re";
                    }
                    
                    var client = new Client()
                    {
                        ClientID = clientId,
                        ServerName = serverName,
                        Text = textUpdated
                    };
                    
                    yield return client;
                    
                }
            }
        }                       
    }   
}

private void BulkUpdateClients(SqlConnection connection, IEnumerable<Client> clients)
{
    const string dropTempTable = "IF OBJECT_ID('[tempdb].[dbo].[##Client]') IS NOT NULL DROP TABLE [tempdb].[dbo].[##Client];";
    
    // drop temp table if exists
    ExecuteSql(connection ,dropTempTable);
    
    // create the temp table
    ExecuteSql($"SELECT TOP 1 [ClientID], [ServerName], [Text] INTO [tempdb].[dbo].[##Client] FROM [dbo].[Client];");

    // copy rows to the temp table
    ExecuteBulkCopy(connection, clients , "[tempdb].[dbo].[##Client]", new[] { "ClientID", "ServerName", "Text" });

    // Use UPDATE JOIN
    ExecuteSql("UPDATE t1 SET [ServerName] = t2.[ServerName], [Text] = t2.[Text] FROM [dbo].[Client] t1 JOIN [tempdb].[dbo].[##Client] t2 ON t1.[ClientID] = t2.[ClientID];");

    // drop temp table
    ExecuteSql(connection,dropTempTable);
    
}

public void BulkUpdateClients()
{
    try
    {
        using(var connection = new SqlConnection(strConn))
        {
            connection.Open();
            
            var clients = GetUpdatedClients(connection);
            
            // it's important to use the same connection and keep it a live
            // otherwise the temp table will be dropped.
            BulkUpdate(connection, clients);                
        }
        
    }
    catch(Exception ex)
    {
        throw ex;
    }
}

If you don't need to use temp table, you can change it to a permanent table (just change the temp table name).