JOIN VS SUBQUERY - mysql

Question

I need to do this but with a subquery, not a join. My problem is, how can I use a subquery to display another column? I could grab the info from there, but I'll be missing the order_date column from the orders table. Can I use a subquery to display it?
SELECT CONCAT(c.customer_first_name, ' ' , c.customer_last_name) AS customer_name, MAX(o.order_date) AS recent_order_date
FROM customers AS c
JOIN orders AS o
ON c.customer_id = o.customer_id
GROUP BY customer_name
ORDER BY MAX(o.order_date) DESC

Answer 1

It's not at all clear what resultset you are trying to return, but it looks an awful like the like the ubiquitous "latest row" problem.
The normative pattern for the solution to that problem is to use a JOIN to the inline view. If there's not a unique constraint, you run the possibility of returning more than one matching row.
To get the latest order (the row in the orders table with the maximum order_date for each customer, assuming that the (customer_id, order_date) tuple is unique, you can do something like this:
SELECT o.*
FROM ( SELECT n.customer_id
, MAX(n.order_date) AS latest_order_date
FROM orders n
GROUP BY n.customer_id
) m
JOIN orders o
ON o.customer_id = m.customer_id
AND o.order_date = m.latest_order_date
If you want to also retrieve columns from the customers table based on the customer_id returned from orders, you'd use a JOIN (not a subquery)
SELECT CONCAT(c.customer_first_name,' ',c.customer_last_name) AS customer_name
, c.whatever
, o.order_date AS recent_order_date
, o.whatever
FROM ( SELECT n.customer_id
, MAX(n.order_date) AS latest_order_date
FROM orders n
GROUP BY n.customer_id
) m
JOIN orders o
ON o.customer_id = m.customer_id
AND o.order_date = m.latest_order_date
JOIN customers c
ON c.customer_id = o.customer_id
ORDER BY o.order_date DESC, o.customer_id DESC
As I mentioned before, if a given customer can have two orders with the exact same value for order_date, there's potential to return more than one order for each customer_id.
To rectify that, we can return a unique key from the inline view, and use that in the join predicate to guarantee only a single row returned from orders.
(NOTE: this approach is specific to MySQL, with this syntax, other RDBMS will throw an error that essentially says "the GROUP BY must include all non-aggregates". But MySQL allows it.)
SELECT CONCAT(c.customer_first_name,' ',c.customer_last_name) AS customer_name
, c.whatever
, o.order_date AS recent_order_date
, o.whatever
FROM ( SELECT n.customer_id
, MAX(n.order_date) AS latest_order_date
, n.order_id
FROM orders n
GROUP BY n.customer_id
) m
JOIN orders o
AND o.customer_id = m.customer_id
AND o.order_date = m.latest_order_date
AND o.order_id = n.order_id
JOIN customers c
ON c.customer_id = o.customer_id
ORDER BY o.order_date DESC, o.customer_id DESC

Answer 2

I am not really sure i understand your question, but i think this works... (not tested though...)
SELECT
(
SELECT
CONCAT(c.customer_first_name, ' ' , c.customer_last_name)
FROM
customers c
WHERE
c.customer_id = o.customer_id
LIMIT 1
) AS customer_name,
MAX(o.order_date) AS recent_order_date
FROM
orders o
GROUP BY
customer_name
ORDER BY
MAX(o.order_date) DESC