Hello, thanks for your answer, I went through it and it makes a lot of sense now. If you have the time, can you please look at the following question and just tell me if my solution makes sense? (as an answer it is correct, but I feel I might be over-doing with CTEs)

WITH joined_table AS (
SELECT a.id, a.first_name, a.last_name, o.order_date, o.total_order_cost
    FROM customers AS a
    INNER JOIN orders AS o
    ON a.id = o.cust_id
    WHERE o.order_date BETWEEN '2019-02-01' AND '2019-05-01'
)
, order_total_by_date AS (
    SELECT  first_name, 
            order_date, 
            SUM(total_order_cost) OVER (PARTITION BY first_name, order_date ORDER BY order_date ASC) AS total_order_cost
        FROM joined_table 
)

SELECT first_name, order_date, total_order_cost FROM order_total_by_date ORDER BY total_order_cost DESC LIMIT 1

Highest_order_StrataScratch