我注意到,當做一個多JOIN查詢,我的查詢不起作用,除非我給一個表名別名。何時需要在SQL中爲表名提供別名?
這裏有一個簡單的例子來說明這一點:
這不工作:
SELECT subject
from items
join purchases on items.folder_id=purchases.item_id
join purchases on items.date=purchases.purchase_date
group by folder_id
這確實:
SELECT subject
from items
join purchases on items.folder_id=purchases.item_id
join purchases as p on items.date=p.purchase_date
group by folder_id
有人能解釋一下嗎?
@Siva修正,感謝 –