MySQL/PHP get average of past two months based on variable month - php

Question

I'm trying to get an average of a value for the past 2 months, but not based on CURDATE()/NOW(). It would be dependent on what month the user was looking at in the application:
Ex:
If I'm looking at May, I would want the average of April and March.
If I'm looking at February, I would want the average of January and December (of the previous year).
I have a function that accepts the month and year of the page the user is on (it also accepts the emp_id, but that is irrelevant for this question).
public function getProtectedAmt($month,$year,$id){
$query = "SELECT avg(total_payout) as avg_payout FROM saved_plan_data WHERE emp_id = '$id' AND //this is where i dont know what to query for";
$result = mysql_query($query) or die(mysql_error());
$row = mysql_fetch_assoc($result);
return $row['avg_payout'];
}
In the table saved_plan_data, there are fields plan_month and plan_year that store INT values of what month/year the employee's total_payout is saved for.
How do I write my WHERE clause to get the AVG for the previous 2 months, depending on what the values of $month and $year are?

Answer 1

I think the easier and more readable solution is this one:
SELECT ...
FROM ...
WHERE
(plan_year, plan_month) IN ((2012, 12), (2013, 1))
You just need to compute the appropriate values.
This is also quite readable:
WHERE CONCAT_WS('-', plan_year, plan_month) IN ('2012-12', '2013-01')

Answer 2

In the function, create two variables $q_month & $q_year.
Set these as comma separated past two months. E.g. if currently the date is May 22, 2013, set the values to $q_month = 4,3 and $q_year = 2013
Use these variables in your query as WHERE Month in ($q_month) and Year in ($q_year)

Answer 3

Edit: Mis-read the question. This would get you the average for the last two months combined.
You could either be very verbose saying where (month and year) or (month and year)
SELECT avg(total_payout) as avg_payout
FROM saved_plan_data
WHERE emp_id=$id
AND (
(plan_month = $month1 AND plan_year = $year1)
OR (plan_month = $month2 AND plan_year = $year2)
)
Or you can cast the plan_month and plan_year as a date and check if the value is between two dates:
SELECT avg(total_payout) as avg_payout
FROM saved_plan_data
WHERE emp_id=$id
AND STR_TO_DATE(CONCAT_WS('-',plan_year,plan_month,'1'),'%Y-%m-%d')
BETWEEN '$year1-$month1-1' AND '$year2-$month2-1'
Also, you really should just store the month and year together. Here is a SO question with good answers about it: mysql datatype to store month and year only

Answer 4

I think the easiest way is to think of the year/month combinations as a number of months since year/month 0. That is, convert to number of months by multiplying the year times 12 and adding the months.
Then the previous two months is easy, and you don't have to worry about year boundaries:
SELECT avg(total_payout) as avg_payout
FROM saved_plan_data
WHERE emp_id = '$id' AND
plan_year*12 + plan_month in ($year*12+month - 1, $year*12+month - 2)