Help with php/mysql if tag already exist

hello there, hope someone can help me with this ...

<?php
 $dbtable  = 'atx_tags';

 $db_conn  = mysql_connect($dbhost, $dbuser, $dbpass) or die ('Error connecting to mysql');
 $db_found = mysql_select_db($dbname);

 if(isset($_POST['submit'])){
  $Tag       = $_POST['Tag'];
  $Date      = date('Y-m-d');

  $query = "INSERT INTO " . $dbtable . " (Tag,Date) VALUES ('$Tag','$Date')";
  mysql_query($query);

 }

?>

<form action="" method="post">
 <input name="Tag" class="input" type="text"><br>
 <input type="submit" name="submit" class="input2" value="Submit">
</form>

PHP:

how i can make if Tag is already added on atx_tags to dont add again and to show echo error video already added ! ... hope someone can help me with this ...

 

make select query
if now exist just insert

 

can you give me the full command please ... im kinda noob with this ...

 

can someone help me ? thanks

 

It will be something like this: Not tested though

<?php
 $dbtable  = 'atx_tags';

 $db_conn  = mysql_connect($dbhost, $dbuser, $dbpass) or die ('Error connecting to mysql');
 $db_found = mysql_select_db($dbname);

 if(isset($_POST['submit'])){
  $Tag       = $_POST['Tag'];
  $Date      = date('Y-m-d');
 }

$result = mysql_query("SELECT Tag FROM " . $dbtable . " WHERE Tag='$Tag';") or die(mysql_error());
if(mysql_num_rows($result) == 0) {
      $query = "INSERT INTO " . $dbtable . " (Tag,Date) VALUES ('$Tag','$Date')";
      mysql_query($query);
    } else {
    echo "Tag already exist, please try again";
 }

?>

PHP:

 

works thank you !!! you save my day !!!