SQL update row if key exists else update a default row - php

Question

Here is what I am trying to do (in pseudo code)
if (dB entry exists) {
UPDATE the dB entry
}else{
UPDATE a default dB entry
}
This is not a "INSERT INTO ... ON DUPLICATE" question.
I'm hoping there is some kind of UPDATE ... ON DUPLICATE type of slick code to do this in one line.
My code creates $userName from the $_GET['U'] request. If $userName exists in the database (it's a unique key), then increment a counter in the database. Else, increment a counter for a default entry.
Here is my current code to update the counter:
$userName = $_GET['U'];
$sql = "UPDATE `stats` SET `count`= `count` + 1 WHERE `userName` = '" . $userName . "'";
mysqli_query($conn,$sql);
And if this particular username doesn't exist, I want this to happen:
$sql = "UPDATE `stats` SET `count`= `count` + 1 WHERE `userName` = 'default'";
mysqli_query($conn,$sql);

Answer 1

Are you looking for this,
$userName = $_GET['U'];
$query_uname_exists = "SELECT count(*) FROM stats WHERE `userName` = '" . $userName . "'";
$uname_count = mysqli_query($conn,$query_uname_exists);
if($uname_count > 0){
#update
$sql = "UPDATE `stats` SET `count`= `count` + 1 WHERE `userName` = '" . $userName . "'";
mysqli_query($conn,$sql);
}
else{
#update default
$sql = "UPDATE `stats` SET `count`= `count` + 1 WHERE `userName` = 'default'";
mysqli_query($conn,$sql);
}

Answer 2

Use the following query
$sql = "Update stats SET count = count + 1 where username = (case when username= ". $userName ". then username else 'default' end)";

Answer 3

This is the most I can shrink it for you:
$userName = mysqli_query($conn, "SELECT count(*) FROM stats WHERE `userName` = '".$_GET['U']."' LIMIT 1") == 0 ? 'default' : $_GET['U'];
mysqli_query($conn, "UPDATE `stats` SET `count`= `count` + 1 WHERE `userName` = '".$userName."' LIMIT 1");