In Listing 5, you have a standard primary key column called PO_ID (that is, purchase order ID).

在清单5中，您有一个称为PO_ID的标准主键列(即订单id)。

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A join is performed inside the XQuery to show the name of each product, since the purchase order only stores product id, price, and quantity.

在XQuery内执行联接以显示每个产品的名称，因为采购订单仅存储产品ID、价格和数量。

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We could have also used the customer id as a group id to find associations according to the whole customer purchase history.

我们也可以使用顾客id作为组的id来根据顾客购物的完整历史寻找关联。

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When you're ready to buy something from a site, you just drop your ID and credit card on the purchase page.

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