Validate textbox value


I have a textbox nd a submit button on my .aspx page.

Is there a way i can validate if the value entered in the textbox exist in a table?  I would like to do this before the user clicks on the submit button.

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joan2006Connect With a Mentor Author Commented:
Thanks guys but i used javascript, onkeypress and attribute.add functionality.  

Thanks again.
add some code to your page...with the event onChange on the text box..make the code run a function on the page that:

opens the database
runs the following sql

select * from <Table> where <form field> = <table>.field
if record set is empty..
do something.

close recordset
close db
Good fit for an AJAX call.
onChange invoke an AJAX function to return true/ false from your DB. Then alert the user if it fails.
PS : The response will be as fast as your DB query here.. SO it better be a simple lookup in DB
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Why not make it a drop down (<select>) instead?  You would populate that drop down with the existing values in your table.  That way, the validation is inherently done for you, since it's now impossible to enter a value that doesn't exist in the db.
I have done this one php you change this in ASP.NET


      // After enter the value in the textbox called through Ajax
      if(isset($_REQUEST['value'])) {
            $value = $_REQUEST['value'];
            if($value != '') {
                  $valuequery  = "SELECT * FROM users where UserName = '$value'";
                  $valueresult = mysql_query($valuequery);
                  $valuerow = mysql_num_rows($valueresult);

                  // display the correct value for the value
                  if($valuerow > 0) {
                        echo "<input type=text name=test value='' onchange='check_database();'>";
                        echo "<br><font color=red><b>Value is in database</b></font>";
                  else {
                        echo "<input type=text name=test value='$value' onchange='check_database();'>";
                        echo "<br><font color=green><b>Value is not in database</b></font>";
      <!-- Ajax common script it is used in realnigerian copy from that -->
      <script src="scripts/prototype.js" type="text/javascript"></script>
            function checkform() {
                        if(test.value == '') {
                              alert('Please enter some value');
                              return false;
                  return false;

            function check_database() {
                  // In which the textbox is display ("id value for the td textbox")
                  var target = 'textvalue';
                  var url = 'validate.php';
                  var params = 'value='+document.validateform.test.value;
                  new Ajax.Updater(target,url,{ parameters:params });
<form name="validateform" action="validate.php" method="post">
                  <td id="textvalue"><input type="text" name="test" value="" onchange="check_database();"></td>
                  <td><input type="button" name="submit" value="Submit" onclick="return checkform();"></td>
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