MySQL longblob файл конвертировать / сохранять в физическое местоположение [дубликат]

используйте этот пример и измените базу данных SQL на вашу базу данных, а также инструкцию Select

SqlConnection pubsConn = new SqlConnection("Data Source=localhost;Integrated Security=SSPI;Initial Catalog=pubs;");
SqlCommand logoCMD = new SqlCommand("SELECT pub_id, logo FROM pub_info", pubsConn);

FileStream fs;                          // Writes the BLOB to a file (*.bmp).
BinaryWriter bw;                        // Streams the BLOB to the FileStream object.

int bufferSize = 100;                   // Size of the BLOB buffer.
byte[] outbyte = new byte[bufferSize];  // The BLOB byte[] buffer to be filled by GetBytes.
long retval;                            // The bytes returned from GetBytes.
long startIndex = 0;                    // The starting position in the BLOB output.

string pub_id = "";                     // The publisher id to use in the file name.

// Open the connection and read data into the DataReader.
pubsConn.Open();
SqlDataReader myReader = logoCMD.ExecuteReader(CommandBehavior.SequentialAccess);

while (myReader.Read())
{
  // Get the publisher id, which must occur before getting the logo.
  pub_id = myReader.GetString(0);  

  // Create a file to hold the output.
  fs = new FileStream("logo" + pub_id + ".bmp", FileMode.OpenOrCreate, FileAccess.Write);
  bw = new BinaryWriter(fs);

  // Reset the starting byte for the new BLOB.
  startIndex = 0;

  // Read the bytes into outbyte[] and retain the number of bytes returned.
  retval = myReader.GetBytes(1, startIndex, outbyte, 0, bufferSize);

  // Continue reading and writing while there are bytes beyond the size of the buffer.
  while (retval == bufferSize)
  {
    bw.Write(outbyte);
    bw.Flush();

    // Reposition the start index to the end of the last buffer and fill the buffer.
    startIndex += bufferSize;
    retval = myReader.GetBytes(1, startIndex, outbyte, 0, bufferSize);
  }

  // Write the remaining buffer.
  bw.Write(outbyte, 0, (int)retval - 1);
  bw.Flush();

  // Close the output file.
  bw.Close();
  fs.Close();
}

// Close the reader and the connection.
myReader.Close();
pubsConn.Close();