【单选题】【消耗次数：1】
从关系模式中指定若干个属性组成新的关系的运算称为
①
联接
②
投影
③
选择
④
排序
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【简答题】
关系的五种基本运算是并、差、[填空]、选择和投影。
【简答题】
CPU从主存取出一条指令并执行该指令的全部时间叫做[填空1]，它通常包含若干个[填空2]，而后者又包含若干个[填空3]。
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