SQL -- Get items responsible for top 50% of sales - sql

Question

I have a table like this:
ITEM_SALES
ITEM_NAME SALES
Item_name_1 5000
...
Item_name_x 3
What I want to get is get the items that represent the top 50% of sales. So for example, if total sales was 10,000, just item_name_1 alone would represent 50% of sales.
I can obviously get total sales with:
select sum(sales) from ITEM_SALES.
...and then divide by 2 to get how many sales 50% of sales is.
However, I don't know how I'd go from there to getting the top items that represent 50% of sales.

Answer 1

You can do this using analytic functions:
select s.*
from (select item_name, sum(sales) as sumsales,
sum(sum(sales)) over (order by sum(sales) desc) as cumesales,
sum(sum(sales)) over () as totsales,
from item_sales
group by item_name
) s
where (cumesales - sumsales) < 0.5 * totsales;
The subquery calculates the sales for each item, as well as two other values:
The cumulative sales, from highest to that item.
The total sales.
The where clause then gets items up to and include the one that passes the 50% threshold.

Answer 2

Oracle Setup:
CREATE TABLE ITEM_SALES ( ITEM_NAME, SALES ) AS
SELECT 'item_name_' || LEVEL, 50 - 3 * (LEVEL - 1)
FROM DUAL
CONNECT BY LEVEL <= 16;
Query:
SELECT *
FROM (
SELECT ITEM_NAME,
SALES,
SUM( SALES ) OVER ( ORDER BY SALES DESC ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW ) AS cumulative_sales,
SUM( SALES ) OVER ( ORDER BY SALES DESC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING ) AS total_sales
FROM ITEM_SALES
)
WHERE cumulative_sales <= total_sales/2;
Results:
ITEM_NAME SALES CUMULATIVE_SALES TOTAL_SALES
------------ ----- ---------------- -----------
item_name_1 50 50 440
item_name_2 47 97 440
item_name_3 44 141 440
item_name_4 41 182 440
item_name_5 38 220 440