How to get the most sold Product in PostgreSQL? - sql

Question

Given a table products
pid
name
123
Milk
456
Tea
789
Cake
...
...
and a table sales
stamp
pid
units
14:54
123
3
15:02
123
9
15:09
456
1
15:14
456
1
15:39
456
2
15:48
789
12
...
...
...
How would I be able to get the product(s) with the most sold units?
My goal is to run a SELECT statement that results in, for this example,
pid
name
123
Milk
789
Cake
because the sum of sold units of both those products is 12, the maximum value (greater than 4 for Tea, despite there being more sales for Tea).
I have the following query:
SELECT DISTINCT products.pid, products.name
FROM sales
INNER JOIN products ON sale.pid = products.pid
INNER JOIN (
SELECT pid, SUM(units) as sum_units
FROM sales
GROUP BY pid
) AS total_units ON total_units.pid = sales.pid
WHERE total_units.sum_units IN (
SELECT MAX(sum_units) as max_units
FROM (
SELECT pid, SUM(units) as sum_units
FROM sales
GROUP BY pid
) AS total_units
);
However, this seems very long, confusing, and inefficient, even repeating the sub-query to obtain total_units, so I was wondering if there was a better way to accomplish this.
How can I simplify this? Note that I can't use ORDER BY SUM(units) LIMIT 1 in case there are multiple (i.e., >1) products with the most units sold.
Thank you in advance.

Answer 1

Since Postgres 13 it has supported with ties so your query can be simply this:
select p.pId, p.name
from sales s
join products p on p.pid = s.pid
group by p.pId, p.name
order by Sum(units) desc
fetch first 1 rows with ties;
See demo Fiddle

Answer 2

Solution for your problem:
WITH cte1 AS
(
SELECT s.pid, p.name,
SUM(units) as total_units
FROM sales s
INNER JOIN products p
ON s.pid = p.pid
GROUP BY s.pid, p.name
),
cte2 AS
(
SELECT *,
DENSE_RANK() OVER(ORDER BY total_units DESC) as rn
FROM cte1
)
SELECT pid,name
FROM cte2
WHERE rn = 1
ORDER BY pid;
Working example: db_fiddle link