How to subtract from two tables in php mysql using variables

Question

i have this total from my first table (income) and I would want to subtract it from the another table(exp) query. All these code are on thesame page. So how can i use these query variables to subtract and echo result on thesame page but in another location? Query 1

<?php 
include("db.php");
$query = "SELECT source, SUM(income.inamount) FROM income"; 
$result = mysql_query($query) or die(mysql_error());

// Print out result
while($row = mysql_fetch_array($result)){
echo "Total Sales  = N". $row['SUM(inamount)'];
echo "<br />";
}
?>


<?php
include("db.php");
$query = "SELECT amount, SUM(exp.amount) FROM exp"; 

$result = mysql_query($query) or die(mysql_error());

// Print out result
while($row = mysql_fetch_array($result)){
echo "Total Expenditure  = N". $row['SUM(amount)'];
echo "<br />";
}
?>

Answer 1

Check that the data type is set to int.

$myVar = $row['SUM(inamount)'] - $row['SUM(amount)'];
echo $myVar;

Another approach is to assign the two values to their own variable then perform the calculation:

$totalSales = $row['SUM(inamount)'];
$expense = $row['SUM(amount)'];

$actual = $totalSales - $expense;

echo $actual;

Using var_dump() will tell you what values are actually being held in the $row Variables. This will give you clues as to whether the data is getting pulled from the database into the variables properly.

var_dump($row['SUM(inamount)']);
var_dump($row['SUM(amount)']);

Another thing to check is if the actual variables are being set so overall the solution might look like this:

<?php 
include("db.php");

//query 1
$query_income = "SELECT source, SUM(income.inamount) FROM income"; 
$result_income = mysql_query($query_income) or die(mysql_error());
while($row = mysql_fetch_array($result_income)){
$inamount = $row['inamount'];
}

//query 2
$query_exp = "SELECT amount, SUM(exp.amount) FROM exp"; 
$result_exp = mysql_query($query_exp) or die(mysql_error());
while($row = mysql_fetch_array($result_exp)){
$amount = $row['amount'];
}

if(isset($inamount, $amount))  {
 $actual = $amount - $inamount;
 echo $actual // obviously you format $actual to your preferred     output
 }
 else {
   echo "Variable data not set";
 }
?>

Answer 2

Assign the value of $row['SUM(inamount)'] to a variable after echoing total sales, which you can use to perform the subtraction:

...
$var=$row['SUM(inamount)'];
...
//after printing total expenditure
echo "Total Profit  = ". ($var-$row['SUM(amount)']);

2 observations, though: 1. mysql libary is already deprecated, use mysqli or PDO instead. 2. There is no need to include db.php twice, if the 2 pieces of code are in the same page.

Answer 3

My best suggestion should be to make just one query and process the substraction within the query, you can make it as follow:

<?php 
include("db.php");
$query = "SELECT inc.source, SUM(inc.inamount) as in_amount, SUM(exp.amount) as out_amount, SUM(inc.inamount)-SUM(exp.amount) as total FROM income as inc, exp"; 
$result = mysql_query($query) or die(mysql_error());

// Print out result
while($row = mysql_fetch_array($result)){
echo "Total Sales  = N". $row['in_amount'];
echo "<br />";
echo "Expenditure Sales  = N". $row['out_amount'];
echo "<br />";
echo "Total after substraction  = N". $row['total'];
echo "<br />";
}
?>

But if you strictly want to make the substraction with the variables, then you can make it as follow:

<?php 
include("db.php");
$query = "SELECT source, SUM(inamount) as in_amount FROM income"; 
$result = mysql_query($query) or die(mysql_error());

// Print out result
while($row = mysql_fetch_array($result)){
$total_sales = $row['in_amount'];
echo "Total Sales  = N". $row['in_amount'];
echo "<br />";
}
?>

<?php
include("db.php");
$query = "SELECT SUM(amount) as out_amount FROM exp"; 

$result = mysql_query($query) or die(mysql_error());

// Print out result
while($row = mysql_fetch_array($result)){
$total_expenditure = $row['out_amount'];
echo "Total Expenditure  = N". $row['out_amount'];
echo "<br />";
}
?>

<?php
$total = $total_sales - $total_expenditure;
echo "Total after substract = N". $total;
echo "<br />";
?>

Is up to you what option you would like to use, but i suggest not to make two different queries to get two values, because they could change within a timeframe.