SQL - GROUB BY - HAVING - MISSING ROWS - mysql

Question

the following is the situation. I need to connect an order-table with a message-table. But i'm only interested in the first message(lowest message-id). The connection between the tables is the orderid.
$result = $this->db->executeS('
SELECT o.*, c.iso_code AS currency, s.name AS shippingMethod, m.message AS note
FROM '._DB_PREFIX_.'orders o
LEFT JOIN '._DB_PREFIX_.'currency c ON c.id_currency = o.id_currency
LEFT JOIN '._DB_PREFIX_.'message m ON m.id_order = o.id_order
LEFT JOIN '._DB_PREFIX_.'carrier s ON s.id_carrier = o.id_carrier
LEFT JOIN jtl_connector_link l ON o.id_order = l.endpointId AND l.type = 4
WHERE l.hostId IS NULL AND o.date_add BETWEEN DATE_SUB(NOW(), INTERVAL 1 WEEK) AND NOW()
GROUP BY o.id_order
HAVING MIN(m.id_message)
LIMIT '.$limit
);
This query works so far. But now orders without a message are missing.
Thank you for your help!
Markus

Answer 1

You want to select several orders and per order the first message. This is generally difficult in MySQL for the lack of window functions (e.g. ROW_NUMBER OVER). But as it's just one column from the message table you are interested in, you can use a subquery in the SELECT clause.
SELECT
o.*,
c.iso_code AS currency,
s.name AS shippingMethod,
(
SELECT m.message
FROM message m
WHERE m.id_order = o.id_order
ORDER BY m.id_message
LIMIT 1
) AS note
FROM orders o
JOIN currency c ON c.id_currency = o.id_currency
JOIN carrier s ON s.id_carrier = o.id_carrier
WHERE o.date_add BETWEEN DATE_SUB(NOW(), INTERVAL 1 WEEK) AND NOW()
AND NOT EXISTS
(
SELECT *
FROM jtl_connector_link l
WHERE l.endpointId = o.id_order
AND l.type = 4
);