Getting current total and previous one from MySQL

As the part of the Best Webdesign, we ever get the request from our customer to show the total selling in a month including previous selling to count the percentage of profit..

To do that, we often use a very simple way, namely using two queris of MySQL. It works well as we need, but it is not too effective.
The way we often use is like below:

<?php
$result=doQuery("select t1.total,t1.tanggal from total_realisasi t1
where LEFT(t1.tanggal,7)=? order by t1.tanggal DESC", Array('2019-07'));

foreach(doArray($result) as $row)
	{
		$result2=doQuery("SELECT t2.total FROM total_realisasi t2 WHERE t2.tanggal < ? order by t2.tanggal DESC limit 1", Array($row['tanggal']));
	}
?>

doQuery and doArray is sql query according to mysql class we us.

Actually, the same result can be get just with one MySQL query.
To do so, we can run query as follows:

<?php
$result=doQuery("select 
t1.total,t1.tanggal,
IFNULL((SELECT t2.total FROM total_realisasi t2 WHERE t2.tanggal < t1.tanggal order by t2.tanggal DESC limit 1), 0) as prevjml
from total_realisasi t1
where LEFT(t1.tanggal,7)=? order by t1.tanggal DESC", Array('2019-07'));
?>

IFNULL((SELECT t2.total FROM total_realisasi t2 WHERE t2.tanggal < t1.tanggal order by t2.tanggal DESC limit 1), 0) as prevjml means that if there is no previous date found, the result will be 0.
LEFT(t1.tanggal,7) is MySQL function to get 7characters from the left.

From the query above, we will get the result as follows:

total  |  tanggal  |  prevjml
6000000   |  2019-07-30   |  84000000
84000000   |  2019-07-29   |  39000000
39000000   |  2019-07-27   |  50000000
50000000   |  2019-07-26   |  5000000
5000000   |  2019-07-18   |  6000000
6000000   |  2019-07-16   |  10000000

We can see that the value of the column of prevjml is the same as the value of the column tanggal at previous date.
For example, the value of the column of prevjml at 2019-07-30 is the value of the column total at  2019-07-29. By using the result above, we can calculate the percentage easily with PHP.

Good luck.