设有如下3个关系表：
下列操作中正确的是( )。
A．T=RRS
B．T=R∪S
C．T＝R×S
D．T=R/S
参考答案：C
解析： R表中只有一个域名A，有两个记录，分别是m和n: s 表中有两个域名，分别是B和C，其所对应的记录分别为1和3。表T是由R的第一个记录依次与S的所有记录组合，然后再由R的第二个记录与S的所有记录组合，形成的一个新表。上述运算符合关系代数的笛卡尔积运算规则。关系代数中，笛卡尔积运算用“×”来表示。因此，上述运算可以表示为T=R×S。
| 完形填空 Several years ago, while attending a communication course, I experienced a most unusual process. The instructor asked us to list 1 in our past that we felt 2 of, regretted, or incomplete about and read our lists aloud. This seemed like a very 3 process, but there's always some 4 soul in the crowd who will volunteer. The instructor then 5 that we find ways to 6 people, or take some action to right any wrong doings. I was seriously wondering how this could ever 7 my communication. Then the man next to me raised his hand and volunteered this story: "Making my 8 , I remembered an incident from high school. I grew up in a small town. There was a Sheriff 9 of us kids liked. One night, my two buddies and I decided to play a 10 on him. After drinking a few beers, we climbed the tall water tank in the middle of the town, and wrote on the tank in bright red paint: Sheriff Brown is a s.o.b.(畜生). The next day, almost the whole town saw our glorious 11 . Within two hours, Sheriff Brown had us in his office. My friends told the truth but I lied. No one 12 found out." "Nearly 20 years later, Sheriff Brown's name 13 on my list. I didn't even know if he was still 14 . Last weekend, I dialed the information in my hometown and found there was a Roger Brown still listed. I tried his number. After a few 15 , I heard, "Hello?" I said, "Sheriff Brown?" Paused. "Yes." "Well, this is Jimmy Calkins." "And I want you to know that I did it!" Paused. "I knew it!" he yelled back. We had a good laugh and a 16 discussion. His closing words were: "Jimmy, I always felt bad for you 17 your buddies got it off their chest, but you were carrying it 18 all these years. I want to thank you for calling me...for your sake." Jimmy inspired me to 19 all 101 items on my list within two years, and I always remember what I learned from the course: It's never too late to 20 the past wrongdoings. | ||||
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