下列关系运算的叙述中，正确的是( )。
A．投影、选择、连接是从二维表行的方向进行的运算
B．并、交、差是从二维表的列的方向来进行运算
C．投影、选择、连接是从二维表列的方向进行的运算
D．以上3种说法都不对
参考答案：C
解析：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)，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。后者是从二维表的列的方向来进行运算的。
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| It was a warm March evening, and I'd just taken a seat on the bus that would take me home. It was nearly 6:00 PM, but the bus was not yet full and the driver gave no 1 that he intended to start the bus. A middle-aged woman took a seat opposite me. She was crying. Not speaking to anyone in particular, she 2 narrated her story. She had come to the city to visit her daughter. On the way to the terminal, a thief had 3 one of her bags. It had contained half of the money she'd brought with her. The other half was hidden under her blouse, so she 4 still had some money left. The bus conductor, driver, and other passengers listened to her tale. At this time, an old man 5 got on the bus. He sat in the seat directly in front of the woman. After a few minutes, all seats were 6 . The driver got behind the wheel and started the engine. The bus conductor collected tickets and began asking us where we were 7 . When he came to the old man's seat, he became 8 and asked the old man Whether he had any money. The old man 9 that he did not. He explained that he 10 all his money this morning when he'd accidentally got on the wrong bus and now he was trying to go home. Upon heating this, the bus conductor 11 the old man to get off the bus. The old man was almost in tears as he begged the bus conductor to let him take that bus 12 he could get home before dark. The bus driver approached the old man, and repeated the conductor's 13 to get off the bus. "Stop troubling him! Can't you see he's only trying to get home?" she interfered. "He doesn't have any money!" the driver shouted. "Well, that's no 14 to throw him off the bus," she insisted. Then she said, "How much is his fare?" The bus conductor mumbled the amount. "Fine," said the woman. She 15 inside her blouse, took out her 16 money, and handed the fares to the bus conductor. "Here's his fare and mine. Just stop giving him a hard time." All eyes turned 17 the woman, who, just minutes before, had been crying over the money she'd lost. "It's only money," she shrugged. By the time the bus rolled out of the terminal, she had given the old man bread and a dollar. She rode the rest of the way home 18 a Mona Lisa smile of peace and grace, and the money she'd lost 19 was forgotten. On the road of life, the politeness and smiles of 20 can lighten our loads and lift our spirits. How much sweeter the journey when we make it a little smoother and richer for others! | ||||
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