Calculate best sale between several sellers - postgresql

Question

I'm using postgre .
Let's say there are 5 sellers .
Each month sale is recorded inside the database like this ( userId:6, january : 10000$, february:20000$ , march : 10000$ ... ,december:50000$, year :2018 )
I need to calculate , possibily with only one query, the best of each month sale in one array of this format : ( january : 15000$, february:30000$ , march : 40000$ , year :2018 ), i dont need the userId . I simply need to compare each sales per months and display the best amount ...
For now, i've got this code, who works well, givin me the user 6 sales per month on a given year :
SELECT date_trunc('month', date_vente) AS txn_month, sum(prix_vente) as monthly_sum,count(prix_vente) AS monthly_count
FROM crm_vente
WHERE 1=1
AND date_part('year', date_vente) = 2018
AND id_user = 6
GROUP BY txn_month ORDER BY txn_month
I wonder if somebody could tell me what kind of technology i could use to get the best of sales each 12 months between of the 5 employees .
COuld i use view ? SHould i better do a for loop in php, with each of the users sales per months, then do a kind of comparative array ?
No need to give me a full resolution, but maybe an advice on how to do, directly with postgre ? Because my only solution for now is to use php and to do a not nice code .
Nice day, ill check on MOnday
Sorry for my english

Answer 1

WITH monthly_sales AS (
SELECT
date_trunc('month', date_vente) AS txn_month,
user_id,
sum(prix_vente) as monthly_sum,
FROM crm_vente
WHERE 1=1
AND date_part('year', date_vente) = 2018
GROUP BY txn_month, user_id
ORDER BY txn_month, user_id),
rank_monthly_sales_by_user_id AS (
SELECT
*,
ROW_NUMBER() OVER (PARTITION BY txn_month ORDER BY monthly_sum DESC) AS rank
FROM monthly_sales)
SELECT
txn_month,
monthly_sum
FROM rank_monthly_sales_by_user_id
WHERE rank = 1
ORDER BY txn_month ASC;
Firstly what you should do is get the totals per month by user. This is the top subquery called monthly sales. Monthly_sales sums the sales of each user by month
Next, to get the top user for each month in terms of their total sales you have to rank the rows returned by the previous subquery. This is down by ROW_NUMBER()
ROW_NUMBER() gets the row number in a specified window, in this case it's ordering the rows from monthly_sales for each month (it starts ordering again from 1 each month). The PARTITION BY statement is the window in which we want to perform the row count, here it's month since we want to order our user_id's sales by month. The ORDER BY statement says how to order the rows from 1 to n. We're using monthly_sum in descending order. So the highest monthly sum is 1, lowest is 6
The next query is selecting only the rows from rank_monthly_sales_by_user_id that are the top sales for the month (WHERE rank = 1)
This leaves us with a output where is row is a month, with the highest sale for that month
Let me know if that was what you needed help with