我正在尝试运行一个大致等于的汇总查询:
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。
我制作了这个查询表达式(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')
它有两个问题:
ctr
为0.0,因为它在数据库级别划分整数division by zero
什么是正确的解决方案?
答案 0 :(得分:12)
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')
答案 1 :(得分:0)
关于如何扩展它的确切数学取决于你,但在嵌套ExpressionWrapper中添加python常量应该足够了:
ctr = models.ExpressionWrapper(
models.F('clicks')/models.ExpressionWrapper(
models.F('impressions') + 1, output_field = models.FloatField()
),
output_field = models.FloatField()
)
答案 2 :(得分:0)
您可以将django-pg-utils
package用于处理除以零的divide
查询表达式并返回浮点值
from pg_utils import divide
Stats.objects.annotate(ctr=divide(F('clicks'), F('impressions')))