问题 问答题

为了维护数据库的参照完整性,当删除被参照关系的元组时,系统可能采取哪些做法?(15分)

答案

参考答案:为了维护数据库的参照完整性,当删除被参照关系的元组时,系统可能采取如下三种做法(每种做法得5分):   (l) 级联删除(cascades):即,当删除被参照关系的元组时,同时将参照关系中所有外键值与被参照关系中要被删除元组的主键值相等(相对应)的元组一起删除。   (2) 拒绝删除(restricted):即,只当参照关系中没有任何元组的外键值与被参照关系中要被删除的元组的主键值相等(相对应)时,系统才执行该删除操作,否则拒绝执行该删除操作。   (3) 置空值删除(nullfies):即,当删除被参照关系的元组时,同时将参照关系中所有与被参照关系中要被删除元组的主健值相等(相对应)的外键值都置为空值。

单项选择题
阅读理解

阅读理解。

阅读下面短文,并根据短文后面的要求答题。

     Experts are finding your standard first aid response may be the worst thing you can do in an emergency.

Here's how to update your first aid kit.

     CUTS AND SCRAPES

      When the skin isn't broken, it's hard to beat iodine (碘酒) for killing bacteria(细菌). 3. _____________

doctors use it to clean before surgery. But when there's a cut, full - strength iodine and rubbing alcohol can

be harmful to skin cells, preventing healing. Putting iodine on cuts and wounds kills bacteria, but it won't clean

the wound. The most effective way is washing the wound with water.

      BURNS

     If you spread some butter on the burn to ease the pain, it isn't a good idea. Butter was thought to cool the

burn, but it can cause infection and create and environment for bacterial growth. Run the burn under cool

water immediately to help remove the heat and put an end to the damaging process. Wrap the burn with a

bandage, and keep it clean and dry. If blisters (水泡) form, don't break them.

      POISONING

     When a child swallows anything poisonous, parents probably think they're well prepared if ipecac (催吐

剂) is on hand. Wrong. Ipecac was thought to be a good way to treat a child who had swallowed a poisonous

substance, but not any more because it doesn't always remove the toxic substance (有毒物质). That may

cause dehydration (脱水) and prevent doctors from giving other treatments.

      EXCESS BLEEDING

      Everyone learns how to stop bleeding with a tourniquet (止血带). But studies show this method causes

more harm than good. Tourniquets can increase the risk of tissue damage or even the loss of a limb. Apply

direct pressure to the wound. Simply place a clean cloth on the wound and press firmly; don't remove the

cloth, event if it gets black. If necessary, add more cloths. If that's not enough, you can further slow the

blood flow by applying pressure to the main artery (动脉) of the upper arm or leg.

1. What is the best title of the passage? (Please answer within 6 words.)

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2. Which sentence in the passage can be replaced by the following one? However,

    by studying we know this method does more harm instead of doing more good.

________________________________________________________________________________

3. Please fill in the blank with proper words or phrases to complete the sentence.

    (Please answer within 5 words.)

_________________________________________________________________________________

4. Translate the underlined sentence in the passage into Chinese.

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