Grouping values with IF, month-by-month, in a select statement - mysql

Question

I am trying to run a report that will group by month the totals of expense accounts from an expense table which has the followings columns on each row:
expense_acc
debit
credit
post_date
The desired output of the report is in the following column format:
EXP ACC - JAN - FEB - MAR
This is my SQL select query:
SELECT expense_acc,
if(MONTH(post_date)=1,SUM(expenses.debit-expenses.credit),0) AS 'JAN',
if(MONTH(post_date)=2,SUM(debit-credit),0) AS 'FEB',
if(MONTH(post_date)=3,SUM(debit-credit),0) AS 'MAR'
FROM expenses
WHERE YEAR(expenses.entered)='2016'
GROUP BY expenses.expense_acc
The results are not grouping the expense values by month as expected. I am seeing grouping in the first row, regardless of the transaction date.

Answer 1

You have two parts to your requirement.
A month-by-month aggregate of your table's contents
A pivot table rendering, in which you pivot by-month rows to lie in columns.
Also, SUM(expenses.debit-expenses.credit) isn't resilient if the debit or credit columns ever contain NULL values.
Also, YEAR(date) defeats any index on date.
If you're wise you'll handle these requirements in two steps. For one thing, it will be easier to troubleshoot your results. For another, the next person who comes along will better understand your project.
The month-by-month aggregate:
SELECT expense_acc, LAST_DAY(post_date) month_ending,
SUM(expenses.debit) - SUM(expenses.credit) as net_expenses
FROM expenses
WHERE post_date >= '2016-01-01'
AND post_date < '20016-12-31' + INTERVAL 1 DAY
GROUP BY expense_acc, LAST_DAY(post_date)
ORDER BY expense_acc, LAST_DAY(post_date)
This will give you a row for each account and date. The row will show the account, the month-ending date, and the net expenses. (I don't understand expenses.entered in your example. It's best to filter on the same date you use to make your aggregate.) Your auditors will appreciate this separation of logic.
Next, you can use this as a subquery, to make your pivot display.
That's pretty straightforward:
SELECT expense_acc,
SUM(IF(MONTH(month_ending)=1,net_expenses,0)) as jan,
SUM(IF(MONTH(month_ending)=2,net_expenses,0)) as feb,
SUM(IF(MONTH(month_ending)=3,net_expenses,0)) as mar,
...
FROM (
SELECT expense_acc, LAST_DAY(post_date) month_ending,
SUM(expenses.debit) - SUM(expenses.credit) as net_expenses
FROM expenses
WHERE post_date >= '2016-01-01'
AND post_date < '20016-12-31' + INTERVAL 1 DAY
) z
GROUP BY expense_acc
ORDER BY expense_acc
But, you may want to do the pivoting in a client program. It's notoriously hard to write and maintain MySQL pivot code.

Answer 2

SELECT
SUM(JAN) AS JAN,
SUM(FEB) AS FEB
SUM(MAR) AS MAR
FROM
(
SELECT expense_acc,
if(MONTH(post_date)=1,SUM(expenses.debit-expenses.credit),0) AS 'JAN',
if(MONTH(post_date)=2,SUM(debit-credit),0) AS 'FEB',
if(MONTH(post_date)=3,SUM(debit-credit),0) AS 'MAR'
FROM expenses
WHERE YEAR(expenses.entered)='2016'
GROUP BY expenses.expense_acc
)TMP
GROUP BY expense_acc

Answer 3

In my opinion, the optimal way is that:
SELECT expense_acc, MONTH(post_date) month_num, SUM(debit-credit) as total
FROM expenses
WHERE YEAR(expenses.entered) = '2016'
GROUP BY expenses.expense_account, MONTH(post_date)
That will give you something like
|expense account | mont_num | total |
-----------------------------------------
| 2345 | 1 | 50 |
-----------------------------------------
| 2345 | 2 | 30 |
-----------------------------------------
| 2346 | 1 | 45 |
...
Because if you are doing one if statement per month, you will get 12 ifs.
It's better to manage the format of rows in you controller.
Hope it will help you.