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How can I check whether there already is any duplicate entry in the available database? I don't want to prevent it, simply a warning. Thus if it already exists it just gives a warning, and it's up to the user to ignore it or not.

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edited by

You can use the following code:

$key_id = 123;

$result = mysql_query("SELECT * FROM table WHERE key_id='$key_id'");
$num_rows = mysql_num_rows($result);

if ($num_rows) {
   trigger_error('It exists.', E_USER_WARNING);

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