Sum/aggregate data based on dates - sql

Question

I have the following table about the items sold out in every shop.
table columns
shop_id: the specific id of the shop,
sold: the purchase amount as dollars ($)
time: the date and time of the purchase.
data
shop_id sold time
1 12.44 23/10/2014 20:20
1 12.77 24/10/2014 20:18
1 10.72 24/10/2014 20:18
1 14.51 24/10/2014 20:18
2 5.94 22/10/2014 20:11
2 15.69 23/10/2014 20:23
2 8.55 24/10/2014 20:12
2 6.96 24/10/2014 20:18
3 8.84 22/10/2014 20:21
3 7.82 22/10/2014 20:21
3 22.19 23/10/2014 20:23
3 13.21 23/10/2014 20:23
4 14.60 23/10/2014 20:20
4 12.19 23/10/2014 20:23
4 5.41 24/10/2014 20:18
4 10.93 24/10/2014 20:19
5 18.54 22/10/2014 20:21
5 7.48 22/10/2014 20:21
5 10.67 24/10/2014 20:18
5 15.96 24/10/2014 20:18
I have 3 classifiers per purchase :
purchase classifiers
low: 0-8 $
medium: 8-12 $
high: 12 and higher $
What I would like to do is to write a query using PostgreSQL that will produce the total purchase each day by each type of purchase classifier.
desired output
date low medium high
22/10/2014 29.10 14.51 12.77
23/10/2014 0 0 70.06
24/10/2014 16.34 51.24 41.39
Thank you very much in advance for your kind help. I am not experienced with postgreSQL. I would do this in R easily, but since moving a huge database table to R is very cumbersome, I need to learn some postgreSQL.

Answer 1

I believe you can do this using conditional aggregation like this:
select
cast(time as date),
sum(case when sold > 0 and sold <= 8 then sold else 0 end) as low,
sum(case when sold > 8 and sold <= 12 then sold else 0 end) as medium,
sum(case when sold > 12 then sold else 0 end) as high
from your_table
group by cast(time as date);
You might have to tweak the ranges a bit - should 8 fall into both low and medium? Also, I can't remember if cast("time" as date) is the correct syntax for Postgresql - I suspect it might not be, but just replace that part with the correct function to get the date from the datetime/timestamp column. It should probably be "time"::date