User table correction

Language: PHP
//catch variables from form
if($password1==$password2){ // we are sure the person has entered his password correctly twice, so now he should know his password and we and jim can be sure he has entered the one he intended 
     //check if the user exist by doing a sql query
    $sql=... select from db where username=';$username';.....
   $num_rows=mysql_num_rows($sql);
    if($num_rows >1) { 
    echo " This user exists ...try again... "; // and add a link back to registration form 
    } else 
    { // the user does not exist, his password match so do a insert query
    }
} 
else{ //the passwords do not match
echo "The password do not match... try again....";
}