Du musst noch definieren nach was gruppiert werden soll. Schau dir mal
group by an.
Deine Query sollte so das richtige Ergebnis liefern:
Code:
SELECT
pc.`id_product_comment`,
sum(if(hp.helpful=1,1,0)) AS helpful,
IF(c.id_customer, CONCAT(c.`firstname`, ' ', LEFT(c.`lastname`, 1)), pc.customer_name) customer_name,
pc.`content`,
pc.`grade`,
pc.`date_add`,
pc.title
FROM `ps_product_comment` pc
LEFT JOIN `ps_customer` c ON c.`id_customer` = pc.`id_customer`
LEFT JOIN `wr_comment_helpful` hp ON ( pc.`id_product_comment` = hp.`id_product_comment`)
WHERE pc.`id_product` = 11
GROUP BY pc.`id_product_comment`
ORDER BY pc.`date_add` DESC