SQL: calculation on two columns with multiple group by statements - sql

Question

I have a table which has the following columns:
user_id - includes duplicates
product_id - includes duplicates
purchases - number of purchases of given product_id
My table looks somewhat like this:
user_id date product_id purchases
0 1 1 1 4
1 1 2 1 0
2 1 3 2 0
3 1 4 2 0
4 2 1 1 1
5 2 2 1 0
6 2 3 1 1
7 3 1 2 0
8 3 2 3 0
9 4 1 5 1
My goal is to calculate the following metric:
% of products that were purchased at least once, grouped by user
For example: user 1 had 2 products, one of them got purchased at least once, the other one did not get purchased at all. So the metric would be the number of products that got purchased at least once / number of all products per user: 1/2 * 100 = 50%
I have little SQL experience so I do not have any legitimate code that could be corrected.
My desired output would be like this:
user_id total_products products_with_purchases metric
0 1 2 1 50%
1 2 1 1 100%
2 3 2 0 0%
3 4 1 1 100%
I would appreciate seeing a good practice solution to this problem. Many thanks!

Answer 1

select
user_id,
count(distinct product_id) as total_products,
count(distinct case when purchases > 0 then product_id end) as products_with_purchases,
100.00 * count(distinct case when purchases > 0 then product_id end)
/ count(distinct product_id) as metric
from T as t
group by user_id
https://rextester.com/EDSY39439

Answer 2

You can do this all in one query but this is the type of situation where it is easier to understand with sub-queries -- sql optimizer should make it fast.
select
user_id,
total_products,
products_with_purchase,
(products_with_purchase / total_products) * 100 as metric
from (
select -- group by user to get totals
user_id,
count(product_id) as total_products,
sum(case when purchases > 0 then 1 else 0 end) as products_with_purchase
from ( -- group by user and product and get purchase items
SELECT user_id, product_id, sum(purchases) as purchases
FROM table
GROUP BY user_id, product_id
) X
group by user_id
) X2

Answer 3

I Am Mohit Sahni
you can solve the above problem with the below SQL Code:
select
user_id,
count(distinct product_id) as total_products,
sum(case when purchases = 0 then 0 else 1 end) as products_with_purchases,
((sum(case when purchases = 0 then 0 else 1 end))/count(distinct product_id))*100 as metric
from
table
group by
user_id