MySql cumulative total grouped by day on maximum total - mysql

Question

I am trying to calculate a cumulative sum of values that can occur more than once on a given day. Therefore i only want to keep the max value for each given day.
currently i have:
SELECT
DATE_FORMAT(created_at, '%d/%m') AS day1,
SUM(principal) over (order by created_at) AS tot
FROM loan;
OUTPUT:
[
{
"day1" : "21/08",
"tot" : 200
},
{
"day1" : "21/08",
"tot" : 1200
},
{
"day1" : "21/08",
"tot" : 2200
},
{
"day1" : "21/08",
"tot" : 2500
},
{
"day1" : "25/08",
"tot" : 4500
},
{
"day1" : "25/08",
"tot" : 6500
},
{
"day1" : "27/08",
"tot" : 7000
},
{
"day1" : "27/08",
"tot" : 7600
}
]
I know i need to group it by the day, but it needs to take the max value for each group of days. any ideas?
When i group by like so:
SELECT
DATE_FORMAT(created_at, '%d/%m') AS day1,
SUM(principal) over (order by created_at) AS tot
FROM loan
GROUP BY day1;
I Get:
[
{
"day1" : "21/08",
"tot" : 1000
},
{
"day1" : "25/08",
"tot" : 3000
},
{
"day1" : "27/08",
"tot" : 3500
},
{
"day1" : "30/08",
"tot" : 4200
}
]
Which is obviously wrong. Im not sure why it gives that output. But how could i get it to be correct?

Answer 1

You need to nest the sums:
SELECT DATE_FORMAT(created_at, '%d/%m') AS day1,
SUM(SUM(principal)) OVER (ORDER BY MIN(created_at)) AS tot
FROM loan
GROUP BY day1;
I am surprised that your version works at all. It would fail in most databases, because neither principal nor created_at are aggregation keys.

Answer 2

Presumably, you want:
select
date_format(created_at, '%d/%m') as day1,
max(sum(principal) over (order by created_at)) as tot
from loan
group by day1
Actually, you could also express this with distinct, and a slightly different order by clause:
select distinct
date_format(created_at, '%d/%m') as day1,
sum(principal) over (order by date(created_at)) as tot
from loan
I am not a big fan of the way you represent dates; if your data spreads over more than one year, the result will become ambiguous (in the second option), or even wrong (in the first query). I would recommend adding the year to the date representation, or simply date(created_at), which basically truncates the time portion.