需要条件和转换的计算字段的Django查询表达式 [英] Django query expression for calculated fields that require conditions and casting
本文介绍了需要条件和转换的计算字段的Django查询表达式的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我正在尝试运行一个大致相等于以下的聚合查询:
I am trying to run an aggregation query that is roughly equals to:
select
sum(impressions) as impressions,
sum(clicks) as clicks,
sum(clicks)/sum(impressions) as ctr
from stats
group by product
order by ctr;
使用的数据库是PostgreSQL。
The database used is PostgreSQL.
我做了这个查询表达式(Django 1.9):
I made this query expression (Django 1.9):
Stats.objects.values('product').annotate(
impressions = models.Sum('impressions'),
clicks = models.Sum('clicks'),
ctr = models.ExpressionWrapper(
models.F('clicks')/models.F('impressions')), output_field = models.FloatField()
)
).order_by('ctr')
有两个问题:
-
ctr
是0.0,因为它在数据库级别分割整数 - 它会抛出
除以零
,如果展示次数为0
ctr
is 0.0 because it divides integers at the database level- it throws
division by zero
if impressions are 0
什么是正确的解决方案?
What is the proper solution?
推荐答案
使用条件表达式:
from django.db.models import Case, F, Sum, When
Stats.objects.values('product').annotate(
tot_impressions=Sum('impressions'),
tot_clicks=Sum('clicks')
).annotate(
ctr=Case(When(tot_impressions=0, then=None), # or other value, e.g. then=0
# 1.0*... is to get float in SQL
default=1.0*F('tot_clicks')/F('tot_impressions'),
output_field=models.FloatField())
).order_by('ctr')
这篇关于需要条件和转换的计算字段的Django查询表达式的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!
查看全文