MYSQL查询的一个结果？

Question

我有这张桌子

订单

• orders_id
• order_article_id
• order_invoice_id
• order_customer_id
• order_qty

制品

• articles_id
• article_name
• article_qty
• article_price
• article_amount

客户

• customers_id
• customer_name
• customer_position
• customer_office

发票

• invoices_id
• 日期
• invoice_customer_id

和SQL Join Table

$sql="SELECT customer_name, invoices_id, SUM( order_qty * article_price ) AS amount, article_name
FROM orders
LEFT JOIN articles ON order_article_id = articles_id
LEFT JOIN invoices ON order_invoice_id = invoices_id
LEFT JOIN customers ON order_customer_id = customers_id
WHERE invoices_id =1
GROUP BY customer_name, invoices_id
LIMIT 0 , 30"

这样我得到了结果

• customer_name
• invoices_id
• 金额
• ARTICLE_NAME

问题在于article_name？我有几个文章名称，但我只获得了第一个结果？