如何从子表中获取数据？

SELECT orders.order_id, orders.order_date, order_items.product_name, order_items.quantity
FROM orders
INNER JOIN order_items ON orders.order_id = order_items.order_id;

SELECT order_id, order_date,
       (SELECT product_name FROM order_items WHERE order_items.order_id = orders.order_id LIMIT 1) AS product_name,
       (SELECT quantity FROM order_items WHERE order_items.order_id = orders.order_id LIMIT 1) AS quantity
FROM orders;

SELECT order_id, order_date, NULL AS product_name, NULL AS quantity
FROM orders
UNION ALL
SELECT order_items.order_id, NULL, order_items.product_name, order_items.quantity
FROM order_items;