MySQL不会检索PHP中的第一项

I've now been trying for hour and can't figure the problem out. I've made a php file that fetch all items in a table and retrieves that as JSON. But for some reason after I inserted the second mysql-query, it stopped fetching the first item. My code is following:

...
    case "LoadEntryList":
        $result2 = performquery("SELECT * FROM Entries WHERE Category = '" . $_POST["Category"] .
        "' LIMIT " . $_POST["Offset"] . ", " . $_POST["Quantity"] . "");
        $row2 = $result2->fetch_assoc();
    while($row = $result2->fetch_assoc()) {
        $result3 = performquery("SELECT Username FROM Users WHERE ID = '" . $row2["UserID"] . "'");
        $row3 = $result3->fetch_assoc();
   echo substr(json_encode($row),0,
        strlen(json_encode($row))-1) . ",\"Username\":\"" . $row3["Username"]  . "\"}";
}
...

Any help is greatly appreciated.

EDIT: Thanks for all those super fast responses.

No Need of $row2 = $result2->fetch_assoc();

    <?
    case "LoadEntryList":
    $result2 = performquery("SELECT * FROM Entries WHERE Category = '" . $_POST["Category"] .
        "' LIMIT " . $_POST["Offset"] . ", " . $_POST["Quantity"] . "");
    while($row = $result2->fetch_assoc()) 
    {
        $result3 = performquery("SELECT Username FROM Users WHERE ID = '" . $row["UserID"] . "'");
        $row3 = $result3->fetch_assoc();
        echo substr(json_encode($row),0,strlen(json_encode($row))-1) . ",\"Username\":\"" . $row3["Username"]  . "\"}";
    }
    ?>

Or,

<?
...
    case "LoadEntryList":
    $Category=$_POST["Category"];
    $Offset=$_POST["Offset"];
    $Quantity=$_POST["Quantity"];

    $result3 = performquery("SELECT Entries.*, Users.Username FROM Entries, Users WHERE Entries.Category=$Category AND Entries.UserID=Users.ID LIMIT $Offset, $Quantity");

    $row3 = $result3->fetch_assoc();
    echo substr(json_encode($row),0,strlen(json_encode($row))-1) . ",\"Username\":\"" . $row3["Username"]  . "\"}";
}
...
?>

First you're fetching a row:

$row2 = $result2->fetch_assoc();

Then you start looping at the next row:

while($row = $result2->fetch_assoc()) {

If you want to loop over all of the rows, don't skip the first one. Just loop over all of the rows:

$result2 = // your very SQL-injectable query
while($row2 = $result2->fetch_assoc()) {
  $result3 = // your other very SQL-injectable query
  $row3 = $result3->fetch_assoc();
  // etc.
}

Note that errors like this would be a lot more obvious if you used meaningful variable names. "row2", "result3", etc. are pretty confusing when you have overlapping levels of abstraction.

Important: Your code is wide open to SQL injection attacks. You're basically allowing users to execute any code they want on your database. Please look into using prepared statements and treating user input as values rather than as executable code. This is a good place to start reading, as is this.

I have a addition to David answer(can't comment on it yet)

This line of code:

   $result3 = performquery("SELECT Username FROM Users WHERE ID = '" . $row2["UserID"] . "'");

will always return with the same result. If you were to change $row2[... into $row[... the code would take the rows that get updated by the while loop.

I am not content with the accepted result. The snippet can be fixed / replaced, and also a bad code must be replaced. Also not to mention is that I don't know if anyone spotted a really big mistake in the output. Here is the fix and I'll explain why.

$JSON = array();
$result2 = performquery( '
    SELECT
        e.*, u.Username
    FROM Entries AS e
    LEFT JOIN Users AS u ON u.ID = e.UserID
    WHERE
        e.Category = ' . $_POST['Category'] . '
    LIMIT ' . $_POST['Offset'] . ', ' . $_POST['Quantity'] . '
' );
while( $row2 = $result2->fetch_assoc() ){
    $JSON[] = $row2;
}
echo json_encode( $JSON );

Obviously the main issue is the query, so I fixed it with a LEFT JOIN, now the second part is the output. First it's the way you include the username, and the second what if you had multiple results? Than your output will be:

{"ID":1,"Username":"John"}{"ID":2,"Username":"Doe"}

How do you parse it? So the $JSON part comes in place. You add it to an array and will encode that array. Now the result is:

{["ID":1,"Username":"John"],["ID":2,"Username":"Doe"]}

LE: I left out the sql inject part which as stated by the OP, will be done afterwards? I'm not sure why not do it at the point of writing it, because you may forget later on that you need to sanitize it.