SQL Query to get most recent date of transaction

Question

New to SQL. Can someone please tell me if my SQL query is correct? I am trying to find out how much each unique customer spent on their most recent transaction.

Table Name: Expenditure

Columns:

1. created_time
2. customer_id
3. spend

SELECT
distinct(customer_id), sum(spend)
FROM
Expenditure
WHERE
created_time =
(SELECT MAX(created_time))
FROM
Expenditure
GROUP BY
distinct(customer_id)

Answer 1

If you are using MySQL 8+, then I prefer to use window functions here:

WITH cte AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY created_time DESC) rn
    FROM Expenditure
)

SELECT customer_id, created_time, spend
FROM cte
WHERE rn = 1;

To correct the approach you were trying to use, you can correlate the subquery to the outer query by customer:

SELECT customer_id, created_time, spend
FROM expenditure e1
WHERE created_time = (SELECT MAX(e2.created_time)
                      FROM Expenditure
                      WHERE e2.customer_id = e1.customer_id);

Answer 2

If your using mysql 5.7 below. you might want to try this query.

select distinct(t1.customer_id), sum(t1.spend) 
from Expenditure t1
inner join
    (select max(created_time) as created_time, customer_id) 
    from Expenditure 
    group by distinct(customer_id) t2 on t2.customer_id = t1.customer_id and t2.created_time = t1.created_time
group by 
distinct(t1.customer_id)