如何获取单个mysql值并将其输出到ajax调用？

I'm trying to get a number from a mysql line then outputting it to ajax. the number can't be a string because I will multiply it in ajax. This is what i have so far. I'm not sure what to do from here.

ajax:

$(document).ready(function()
{
    $("#btnCalc").click(function()
    {
        var user     = $("#txtUser").val();
        var amount   = $("#txtAmount").val();
        var category = $("txtCat").val();
        var number   = $("txtNum").val();
        var result = '';

        $.get("code/value.php",
        {
            ID:user,
            amount:amount,
            result:result

        },function(query)
        {
            if ( user > 0 and user < 30 ){
                alert(result);
            }
            else{
                alert( 'invalid user ID');
            }
        });
    });

});

php:

<?php

    $userID = $_GET["ID"];
    $amount = $_GET["amount"];
    $category = $_GET["category"];
    $num      = $_GET["number"];

    require "../code/connection.php";

    $SQL = "select userAmount from user where userID= '$userID'";

    $reply = $mysqli->query($SQL);

    while($row = $reply->fetch_array() )
    {


    }

    if($mysqli->affected_rows > 0){
        $msg= "query successful";
    }
    else{
        $msg= "error " . $mysqli->error;
    }

    $mysqli->close();

    echo $msg;
?>

Pretty straightforward - you just grab the value from the row and cast it as a float.

while($row = $result->fetch_array() )
{
    $msg = floatval($row['userAmount']);
}
if($msg > 0) {
    echo $msg;
} else {
    echo "error" . $mysqli->error;
}

$mysqli->close();

And one small change in your ajax call:

    $.get("code/value.php",
    {
        ID:user,
        amount:amount,
        result:result

    },function(query)
    {
        alert(query);
    });
});

You need to add echo $row['userAmount']; inside or after your while loop, and drop the second echo. You should be able to take result within your AJAX code and use it as a number directly.

Here function(query), query is the response from the AJAX call. So your alert should be:

alert(query);

result is empty.

You also should be using prepared statements and outputting the value you want.

Something like:

<?php
    $userID = $_GET["ID"];
    $amount= $_GET["amount"];
    require "../code/connect.php";
    $SQL = "SELECT userAmount FROM user WHERE userID= ?";
    $reply = $mysqli->prepare($SQL);
    if($mysqli->execute(array($userID))) {
        $row = $reply->fetch_array();
        echo $row['amount'];
    }
    else
    {
        $msg = "error" . $mysqli->error;
    }
$mysqli->close();
?>

Then JS:

$(document).ready(function()
{
    $("#btnCalc").click(function()
    {
        var user       = $("#txtUser").val();
        var amount     = $("#txtAmount").val();
        var result = '';

        $.get("code/value.php",
        {
            ID:user,
            amount:amount,
            result:result

        },function(query)
        {
            alert(query);
        });
    });

});

You can use https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/parseFloat or https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt to convert the value to an integer/float in JS.