PHP- Query MySQLi results nearest a given number - php

Question

I am trying to search for an invoice by the amount. So, I would like to search all invoices +/- 10% of the amount searched, and order by the result closest to the given number:
$search = 100.00
$lower = $search * 0.9; // 90
$higher = $search * 1.1 // 110
$results = $db->select("SELECT ID from `invoices` WHERE Amount >= `$lower` && Amount >= `$higher`");
So, I am not sure how to order these. Let's say this query gives me the following results:
108, 99, 100, 103, 92
I want to order the results, starting with the actual number searched (since it's an exact match), and working out from there, so:
100, 99, 103, 92, 108

Answer 1

You could do this as follows:
$search = 100.00
$deviation = 0.10;
$results = $db->select("
SELECT ID, Amount, ABS(1 - Amount/$search) deviation
FROM invoices
WHERE ABS(1 - Amount/$search) <= $deviation
ORDER BY ABS(1 - Amount/$search)
");
Output is:
+----+--------+-----------+
| id | Amount | deviation |
+----+--------+-----------+
| 3 | 100 | 0 |
| 2 | 99 | 0.01 |
| 4 | 103 | 0.03 |
| 1 | 108 | 0.08 |
| 5 | 92 | 0.08 |
+----+--------+-----------+
Here is an SQL fiddle
This way you let SQL calculate the deviation, by dividing the actual amount by the "perfect" amount ($search). This will be 1 for a perfect match. By subtracting this from 1, the perfect match is represented by the value 0. Any deviation is non-zero. By taking the absolute value of that, you get the exact deviation as a fractional number (representing a percentage), like for example 0.02 (which is 2%).
By comparing this deviation to a given maximum deviation ($deviation), you get what you need. Of course, ordering is then easily done on this calculated deviation.

Answer 2

Try this:
$search = 100.00
$lower = $search * 0.9; // 90
$higher = $search * 1.1 // 110
$results = $db->select("SELECT ID from `invoices`
WHERE Amount >= `$lower` && Amount <= `$higher`
ORDER BY ABS(Amount - $search)
");
The ABS function returns the absolute value of its argument (=> it basically removes the minus from negative numbers). Therefore ABS(Amount - $search) returns the distance from the $search value.
Besides that you should consider using prepared statements. Otherwise your application could be vulnerable to sql injection.