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SQL的基本表约束主要有三种形式:候选键定义、外键定义、______________定义。
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第1空:[ "检查约束" ]
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Passage Three Eating an apple a day doesn' t keep the doctor away, but it does reduce the amount of trips you make to the drug store per year. That’ s according to a new study that investigates whether there' s any truth in the old saying.A team of researchers led by Dr Matthew Davis, of the University of Michigan School of Nursing, asked 8, 399 participants to answer survey questions about diet and health. A total of 753 were apple eaters, consuming at least 149g of raw apple per day. The remaining 7, 646 were classed as non-apple eaters. When both groups answered questions on trips to the doctor and trips to the drug store per year, the apple eaters were found to be 27% less likely to visit the druggist for drugs.Trips to the doctor were not significantly affected by apple consumption, though." Evidence does not support that an apple a day keeps the doctor away. However,the small number of uUS adults who eat an apple a day does appear to use fewer prescription medications," the study concludes.Apple eaters were also found to be less likely to smoke and be more likely to have a higher educational attainment than non- apple eaters. While apples do not compete with oranges, they docontain some immune (免疫的) system- increasing vitaminC,which may be why apple -eaters visit the druggist less. With over 8mg of vi taminC per medium-sized fruit, an apple can provide roughly 14% your daily recommendedintake.Previous studies have also linked apple consumption to a lower risk of Type 2diabetes (二 型糖尿病) , improved lung function and a lower risk of colon (结肠) can cer.
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