Prepared SQL statement to test existence of a value in a row

Question

Excuse my ignorance in SQL. I didn't do much in SQL queries or DB design in general, but I'm eager to learn.
Idea behind: I'm traversing a directory tree (Java, jdbc) (depth first) and want to collect all files
in that tree and put the as BLOBS into the database.
To find out whether a file is the same as another file (possibly in a different path) I calculate the md5sum
(not sure whether it is proof against collisions or if I should use SHA-256 or some combination of md5sum + chksum).
Anyway, let the md5sum be the unique value that makes the identity of a file for now.
My problem: Find the jdbc/SQL statements that let me decide whether the file (md5sum) is already stored in a ROW
upon which I can decide to either do the INSERT or mark the existing file as being present in a different path.
I stumbled across the problem in the moment when I first time tried to INSERT a row and got told that
md5sum gotta be unique.
Any ideas for design and query? The code example below is incomplete. Especially what I'm looking for is
how to construct a prepared SQL statement into which I can inject the variable later found (md5sum)
Whether I should do that by COUNT(*) or a different method?
Christoph
Here is my sketchy code (Netbeans project) under construction:
* To change this template, choose Tools | Templates
* and open the template in the editor.
package testsqlite;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.sql.*;
import java.util.Iterator;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;
* @author chris
public class Main {
* @param args the command line arguments
private static Connection conn;
private static String sql="INSERT INTO part(name,md5sum) VALUES (?,?)";
private static PreparedStatement pstmnt,pstmnt_ifexists ;
private static String sql_ifexists="SELECT COUNT(*) FROM part WHERE md5sum=?"; // ???
public static void main(String[] args) throws Exception{
// TODO code application logic here
System.out.println("Testsuite-Collector\n");
String path="c:/usr/local/www/data/testsuite/mandanten/chris/probanden";
File f = new File(path);
Class.forName ("org.sqlite.JDBC");
conn =
DriverManager.getConnection("jdbc:sqlite:c:/users/chris/testsuite/versionen");
Statement stat = conn.createStatement();
pstmnt = conn.prepareStatement(sql);
pstmnt_ifexists = conn.prepareStatement(sql_ifexists);
// process
Traverse(f);
// watch result
ResultSet rs = stat.executeQuery("select * from part;");
while (rs.next()) {
System.out.print("name=" + rs.getString("name") + " ");
System.out.println("md5sum= " + rs.getString("md5sum"));
rs.close();
conn.close();
private static void Traverse(File f) throws IOException {
File[] files = f.listFiles();
for( File file : files) {
if(file.isDirectory()){
Traverse(file);
else {
try {
if(file.length() > 0){
FileInputStream fis = new FileInputStream( file );
String md5 = org.apache.commons.codec.digest.DigestUtils.md5Hex( fis );
System.out.println(file.length() +" " md5"->"+file.getPath());
// check whether md5sum is already existing
pstmnt_ifexists=?????
pstmnt.setString(1, file.getPath());
pstmnt.setString(2, md5);
pstmnt.executeUpdate();
catch (SQLException e) {
System.out.println("SQL error:" e.getMessage()"\n");
}

Answer 1

And you seriously think that attempting to insert a record and catching the error of it failing is a good design!?!?I certainly do. I agree with Peter here. You can either test for the existence of the record and insert it if absent, or insert the record willy-nilly and catch the exception. The second option is (a) atomic even without transactions and (b) clearly twice as efficient. Same consideration applies to e.g.
if (!map.containsKey(key))
  map.put(key, value);
else
  ; // it was already thereas opposed to
if (!map.put(key, value)
  ; // it was already thereand many other situations of this kind, e.g.
try
  if (InetAddress.isReachable(ip))
    s = new Socket(ip, port);
  else
    ; // host not up
catch (ConnectException exc)
  // the host wasn't up during connect
}as opposed to
try
  s = new Socket(ip, port);
catch (ConnectException exc)
  // host not up
}or
try
  if (file.exists())
    in = new FileInputStream(file);
  else
    ; // it wasn't there
catch (IOException exc)
  // it wasn't there, etc ...
}as opposed to
try
  in = new FileInputStream(file);
catch (FileNotFoundException exc)
  // it wasn't there
}Slight absence of timing window problems in the cases that detect via exceptions, and considerably simpler code. Phobias about how to use exception handling need to be overcome.
I've also had it seriously argued to me that you shouldn't catch EOFExceptions, merely change the format of your file so you 'know' when one is coming up. Sorry to say that I think this sort of thing is complete rubbish. There is a need for an out of band EOF signal with all the readXXX() methods, and this is it.