[单项选择]We can learn from the passage that the two talk shows______. A. have monopolized the talk show circuit B. exploit the weaknesses in human nature C. appear at different times of the day D. are targeted at different audiences
第35题: [单项选择]设有关系R=(A,B,C)。与SQL语句 SELECT DISTINCT A FROM R WHERE B=17 等价的关系代数表达式是( )。 A. пA( B. σB=17( C. пA(σB=17() D. σB=17(пA() 参考答案:C 答案解析:[分析] 本题中SELECT语句的基本功能是查询关系R中,满足条件B=17的所有不重复元组中属性A的值。