目标必须是可用数量加以说明。
用户希望知道某样东西的可用数量以及它们的存放地点。
Users want to know how many of something are available and where they're located.
如果所请求商品在stock表中的可用数量小于订购的数量，那么事务将回滚到neworder保存点。
If the available quantity from the stock table for the requested item is less than the quantity ordered, the transaction rolls back to the neworder savepoint.
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