PHP和MySQL字段的PHP总和回声

Question

I have a database with the follow tables:

sales
expenses
taxes
earnings

When I sell some product it adds a item to sales, the same to expenses while I add expenses, taxes are added automaticly when selling and earnings too. They are on the same database but different tables.

I need to sum those fields together. I do it one by one without problems like this:

<?php
$query = mysqli_query($db, "SELECT SUM(total) AS `total` FROM `sales`");
while($result = mysqli_fetch_assoc($query)){
echo number_format($result['total'],0,',','.');} 
?>

and

<?php
$query2 = mysqli_query($db, "SELECT SUM(expense) AS `expense` FROM `expenses`");
while($result2 = mysqli_fetch_assoc($query2)){
echo number_format($result2['expense'],0,',','.');} 
?>

How do I sum those two and echo a result example:

sales - expense = value ?

Answer 1

<?php
    $query = mysqli_query($db, "SELECT SUM(total) AS `total` FROM `sales`");
    $query2 = mysqli_query($db, "SELECT SUM(expense) AS `expense` FROM `expenses`");
    $total_sales = 0;
    $total_expenses = 0;
    while($result = mysqli_fetch_assoc($query)){
        $total_sales = $total_sales + $result['total'];
        echo number_format($result['total'],0,',','.');
    }
    while($result2 = mysqli_fetch_assoc($query2)){
        $total_expenses = $total_expenses + $result['total'];
       echo number_format($result2['expense'],0,',','.');
   } 
?>

The sum would be $total_sales-$total_expenses.