【5】 是从二维表列的方向进行的运算。
参考答案：关系运算
解析：[命题目的] 本题考查了关系运算的基本知识。[解题要点] 在关系模型的数据语言中，一般除了运用常规的集合运算，(并、交、差、笛卡尔积等)外，还定义了一些专门的关系运算，如投影、选择、连接等。前者是将关系(即二维表)看成是元组的集合，这些运算主要是从二维表的行的方向来进行的；后者主要是从二维表的列的方向来进行的运算。两者统称为关系代数。
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| The steep rise in breast cancer over the past fifty years has been blamed on a variety of causes--from late childbearing to pesticides. Now Belgian scientists have evidence that it may be caused by teenagers drinking fizzy soft drinks instead of weak beer. At a meeting in Brussels last week, Jack Janssens of the Limburg University Centre in Diepenbeek described experiments in which teenagers fasted for 12 hours then drank a 330-millilitre bottle of a popular fizzy drink--he won’t divulge the brand--or a bottle of "table beer", a drink containing 1.1 per cent alcohol that was popular in Belgium before soft drinks came on the market. The beer is still served in Belgian hospitals, especially to new mothers. The focus of the study was insulin, the hormone that regulates blood sugar levels. As expected, insulin levels shot up after teenagers drank the sugar-laden soft drinks, but not after the beer, which although it contains complex carbohydrates has few simple sugars. "What was really a new observation," says Janssens, "was that the fatter the teenager was, the more the insulin went up." The insulin levels did not depend on the level of blood sugar or the total weight of an individual, just obesity. Janssens thinks insulin is a key factor in the development of the human breast, because it influences levels of several other hormones, including growth factor, sex steroids and their inhibitory binding proteins. Inappropriate secretion of insulin at a critical phase of development, he says, "might induce lesions in the breast cells leading to breast cancer later in life". He believes his research suggests a vicious circle in which "soft drinks and high caloric foods in pubertal children brings accumulation of body fat and in turn increases the response of insulin". This could lead to loss of sensitivity to insulin, which may have a continuing effect on breast cells. "A history of weight gain in early adult life is associated with an increased breast cancer risk in Western women," says Janssens. Insulin may be the link. In males, a similar effect may lead to testicular or prostate cancer. As for the beer, Janssens couldn’t detect any alcohol in the teenagers’ blood, which shows that they metabolise it efficiently, he says. While insulin may play a role in cancer, he says more epidemiological research is required before blaming soft drinks. Denise Baxter of Brewing Research International in Nuffield, Surrey, also told the meeting that as well as vitamin B, beer contains other anti-cancer nutrients, including antioxidants. The meeting was organised by the Confederation of European Brewers. |
One’s insulin level is controlled by
A．the level of blood sugar.
B．one’s total weight.
C．one’s over-weight.
D．the level of several other hormones.