#20930: Arithmetic operations on aggregates
-------------------------------------+-------------------------------------
Reporter: debanshuk | Owner: nobody
Type: New feature | Status: new
Component: Database layer | Version: master
(models, ORM) | Resolution:
Severity: Normal | Triage Stage:
Keywords: aggregate, annotate | Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by debanshuk):
Hi Wim,
In the example given in the description, it would. And I guess in case of
aggregates it would always be possible to make the calculations out side
aggregate.
But, '''in case of annotate it would not'''. Well, it would certainly be
possible, but wouldn’t be cleaner, as one would have to do the
calculations for each element in resultant query-set.
Eg. Let modified `Xyz` be:
{{{
class Xyz(models.Model):
user = models.ForeignKey(User)
a = models.IntegerField()
b = models.IntegerField()
}}}
Now if I want to find find percent ratio of sum of all `a` and sum of all
`b` of a `user`, for each `user`. I may write:
{{{
Xyz.objects.values('user').annotate(ratio_percent=Sum('a')/Sum('b')*100)
}}}
Currently, to do this (without writing raw SQL), one would have to do
something like:
{{{
result = Xyz.objects.values('user').annotate(total_a=Sum('a'),
total_b=Sum('b'))
final_result = [{'user': r['user'], 'ratio_percent':
r['total_a']/r['total_b']*100} for r in result]
}}}
'''Also''', If someone want to aggregate over annotation results, like in
the example if I want to find maximum of the ratio_percentage, I may do:
{{{
Xyz.objects.values('user').annotate(ratio_percent=Sum('a')/Sum('b')*100).aggregate(Max('ratio_percent'))
}}}
Currently, I would have to do:
{{{
result = Xyz.objects.values('user').annotate(total_a=Sum('a'),
total_b=Sum('b'))
final_result = max(r['total_a']/r['total_b']*100 for r in result)
}}}
'''In the second one, max calculation in done in python. In the first one
it is done in SQL itself.'''
--
Ticket URL: <https://code.djangoproject.com/ticket/20930#comment:4>
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