Selecting the best-performing sales rep by date - sql

Question

Given two tables, sales_reps and sales:
sales_reps
+--------+-------+
| rep_id | name |
+--------+-------+
| 1 | Tony |
+--------+-------+
| 2 | Jim |
+--------+-------+
| 3 | Laura |
+--------+-------+
| 4 | Sam |
+--------+-------+
sales
+------------+----------+--------+-------------+
| sale_date | sales_id | rep_id | sale_amount |
+------------+----------+--------+-------------+
| 2021-01-01 | 1 | 1 | 2000 |
+------------+----------+--------+-------------+
| 2021-01-01 | 2 | 1 | 4000 |
+------------+----------+--------+-------------+
| 2021-01-01 | 3 | 2 | 3000 |
+------------+----------+--------+-------------+
| 2021-01-01 | 4 | 1 | 1000 |
+------------+----------+--------+-------------+
| 2021-01-01 | 5 | 4 | 5000 |
+------------+----------+--------+-------------+
| 2021-01-02 | 6 | 3 | 10000 |
+------------+----------+--------+-------------+
| 2021-01-02 | 7 | 3 | 10000 |
+------------+----------+--------+-------------+
| 2021-01-02 | 8 | 2 | 4000 |
+------------+----------+--------+-------------+
| 2021-01-02 | 9 | 1 | 6000 |
+------------+----------+--------+-------------+
| 2021-01-02 | 10 | 4 | 2000 |
+------------+----------+--------+-------------+
| 2021-01-03 | 11 | 2 | 8000 |
+------------+----------+--------+-------------+
| 2021-01-03 | 12 | 2 | 1000 |
+------------+----------+--------+-------------+
| 2021-01-03 | 13 | 3 | 4500 |
+------------+----------+--------+-------------+
| 2021-01-03 | 14 | 3 | 4500 |
+------------+----------+--------+-------------+
I want to show how many unique reps made sales by date (easy) and the rep_id and name of the rep who generated the highest total sales amount for each date. If more than one rep generated the same greatest total sales amount for a date, I want to show the lesser rep_id and that rep's name. The results should look like this:
+------------+-------------+----------+----------+
| sale_date | unique_reps | best_rep | rep_name |
+------------+-------------+----------+----------+
| 2021-01-01 | 3 | 1 | Tony |
+------------+-------------+----------+----------+
| 2021-01-02 | 4 | 3 | Laura |
+------------+-------------+----------+----------+
| 2021-01-03 | 2 | 2 | Jim |
+------------+-------------+----------+----------+
Laura and Jim both generated $9,000 in sales on 2021-01-03. But Jim's rep_id is 2, which is less than Laura's rep_id of 3. So Jim is displayed as the rep who generated the greatest sales amount on 2021-01-03.

Answer 1

Postgres has a mode() function, but it doesn't allow you to choose which rep to choose in the case of ties. For that, you can be more explicit:
select distinct on (s.sale_date) s.sale_date,
s.rep_id, sr.name,
count(*) over (partition by s.date) as num_reps
from sales s join
sales_reps sr
on s.rep_id = sr.rep_id
group by s.sale_date, s.rep_id
order by s.sale_date, sum(s.sale_amount) desc, s.rep_id, sr.name;
What is this doing? It is aggregating by the date and sales rep. Because of the distinct on, though, it is only taking one row per date. In this row:
count(*) over (partition by date) counts the number of reps (it is counting the rows after the aggregation).
s.rep_id is chosen based on the order by criteria -- first the most sales, then by the lowest rep id.
sr.name is the name of the sales rep.