如果有值则更新行,否则更新

I have a form where the user inserts data but they can go back to the same page to edit their information. My table structure is: id (auto int index), user id (links to other tables), Doc_Name, Abstract

I have an insert query:

$user_id = intval($_SESSION['user_id']);
$Doc_Name = mysql_real_escape_string($_POST['Doc_Name']);
$abstract = mysql_real_escape_string($_POST['abstract']);

$the_query = sprintf("INSERT INTO `document` (`user_id`,`Doc_Name`,`abstract`) VALUES
('%d','%s','%s')", $user_id, $Doc_Name, $abstract);

However, if their is already a row for this user_id then I want the update query instead:

 mysql_query("UPDATE document SET `Doc_Name` = '$Doc_Name', 'abstract='$abstract'
 WHERE id='$_SESSION[user_id]'") or die(mysql_error());

Also, so the user knows what they entered, I tried to use this echo in the text box but that didn't work either,

<textarea name="Doc_Name" style="width:500px; height:150px" type="text" id="Doc_Name"
value="<? echo $row_settings['Doc_Name']; ?>"  size="300"> </textarea>

You can use INSERT ... ON DUPLICATE KEY UPDATE Syntax

http://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html

INSERT INTO table (a,b,c) VALUES (1,2,3) ON DUPLICATE KEY UPDATE c=c+1;

for the textarea you can use

<textarea name="Doc_Name" style="width:500px; height:150px" type="text" id="Doc_Name" size="300"><? echo $row_settings['Doc_Name']; ?></textarea>

everything between the tags is displayed and editable

EDIT: to the other posters: nice, did not know INSERT ON DUPLICATE

the query:

SELECT * FROM document WHERE id='{$SESSION['user_id']}'

php:

if(mysql_num_rows(mysql_query($query)) > 0) {
    //code to be executed if id exists
}