Why "SELECT DISTINCT" with HSQLDB not working? - select

Question

I have a problem. When I try to use the "Select distinct" in HSQLDB, the stream of results returns me all rows instead of me returning only distinct rows.
I've tried the following syntax:
SELECT DISTINCT FROM ratings UserID order by UserID;
SELECT DISTINCT (UserID) FROM ratings order by UserID;
SELECT DISTINCT UserID FROM ratings;
SELECT DISTINCT (UserID) FROM ratings;
None of them works. What is the problem?
If anyone able to help me I appreciate.
Thank you.
The function code where I perform the action is as follows:
private void readUserFile(String filename) {
try {
//Verify if file users exists
boolean exists = (new File(filename)).exists();
if (!exists) {
//If file users not exists create one, bases on distinct users that exist in ratings table
ResultSet rsUsers = jdbcTemplate.getDataSource().getConnection().createStatement().executeQuery("SELECT DISTINCT UserID FROM ratings order by UserID;");
List<String> users = new ArrayList<String>();
while (rsUsers.next()) {
users.add(String.valueOf(rsUsers.getInt(1)));
}
rsUsers.close();
//Create and write to file
BufferedWriter f = null;
f = new BufferedWriter(new FileWriter(filename));
for (String user : users) {
f.write(user);
f.newLine();
}
f.close();
}
PreparedStatement prstInsert = con.prepareStatement("INSERT INTO users VALUES (?)");
BufferedReader in = new BufferedReader(new FileReader(filename));
int i = 0;
while (true) {
String s = in.readLine();
if (s == null) { // end of file
log.info("Total imported users: " + i);
break;
}
i++;
int userid = Integer.parseInt(s);
prstInsert.setInt(1, userid);
if (i != 0 && Math.round((double) i / 100) == ((double) i / 100)) {
log.info("Imported users: " + i);
}
prstInsert.executeUpdate();
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
}