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概念模型的表示方法最常用的是______，这是P.P.S.Shen于1976年提出的。
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数据字典的实现可以有3种途径：人工过程、自动化过程和______。
More than seven hundred years ago, the Prince of Wales had a very big and brave dog, Gelert. One day the Prince wanted to go hunting with his men. He told his dog to stay at home and look after his baby son. The baby was in a wooden cradle, which was like a small bed.When the Prince came back from hunting, Gelert ran out to meet his master. Then the Prince saw blood on Gelert’s jaws and head."What have you done" The Prince said. He rushed into his home and looked for his baby son. The cradle was lying on its side on the floor. The clothes were torn and there was blood on them."So you killed my son!" the Prince said angrily. "You unfaithful dog!" He took out his sword and killed the dog. Just as Gelert was dying, he managed to bark for a moment. Then the Prince heard a baby call to the dog.The Prince ran out of the house and saw his son lying on the ground unhurt. Near him was a dead wolf. Then the Prince knew that Gelert had defended the baby and killed the wolf.The Prince ran back into the house but he was too late. Gelert was dead. The Prince was very sad indeed. Tears ran down his face when he realized he had killed his faithful friend. After this, the Prince never smiled again. Every morning at dawn, he walked up the mountain so that he could stand by the dog’s grave for a few minutes. Whose dog was Gelert().
After a busy day of work and play, the body needs to rest. Sleep is necessary for good health. During this time, the body recovers from the activities of the previous day. The rest that you get while sleeping enables your body to prepare itself for the next day.There are four levels of sleep, each being a little deeper than the one before. As you sleep, your muscles relax little by little. Your heart beats more slowly, and your brain slows down. After you reach the fourth level, your body shifts back and forth from one level of sleep to the other.Although your mind slows down, from time to time you will dream. Scientists who study sleep state that when dreaming occurs, your eyeballs begin to move more quickly (although your eyelids are closed). This stage of sleep is called REM, which stands for Rapid Eye Movement.If you have trouble falling asleep, some people recommend breathing very deeply. Other people believe that drinking warm milk will help make you drowsy. There is also an old suggestion that counting sheep will put you to sleep! The average number of hours of sleep that an adult needs is ().
DML有两类：一类是______型语言，一类是______型语言。前者的语句不能独立使用而必须______某种语言中使用。而后者可以独立使用，通常可以供用户以______使用和以______使用。
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