问题
单项选择题
如下图所示，两个关系R1和R2，它们进行哪种运算后得到R3
A．交
B．并
C．笛卡尔积
D．连接
答案
参考答案：D
解析：连接操作是在两个关系中，选取属性满足条件的元组生成一个新的关系。
搜题请搜索小程序“爱下问搜题”
如下图所示，两个关系R1和R2，它们进行哪种运算后得到R3
A．交
B．并
C．笛卡尔积
D．连接
参考答案：D
解析：连接操作是在两个关系中，选取属性满足条件的元组生成一个新的关系。
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| At the beginning of the twentieth century, many people thought that the American family was falling apart. A century later, we know that this was not the case. However, although the family is still alive in the United States, its size and shape were very different 100 years ago. In the late 1800s and early 1900s, there were mainly two types of families in the United States: the extended and the nuclear. The extended family usually includes grandparents, parents, and children living under the same roof. The nuclear family consists of only parents and children. Today there are many different kinds of families. Some people live in "traditional" families, that is, a stay-home mother, a working father, and their own biological children. Others live in two-paycheck families, single-parent families, adoptive or foster, families, blended families (where men and women who were married before marry again and combine the children from previous marriages into the new families),child less families, and so on. What caused the structure of the family to change? In the early 1900s the birthrate began to fall and the divorce rate began to rise. Women were suddenly choosing to go to college and take jobs outside the home. In the 1930s and 1940s, many families faced serious financial, or money problems during the Great Depression, when many people lost their jobs. During World War II(1939-1945),5 million women were left alone to take care of their homes and their children. Because many men were at war, thousands of these "war widows" had to go to work outside their home. During the next ten years, the situation changed. There were fewer divorces, and people married at a younger age and had more children than the previous generation. It was unusual for a mother to work outside the home during the years when her children were growing up. Families began leaving cities and moving into single-family homes in the suburbs. The traditional family seemed to be returning. In the years between 1960s and 1990s, there were many important changes in the structure of the family. From the 1960s to the early 1970s, the divorce rate doubled and the birthrate fell by half. The number of single-parent families tripled, and the number of couples living together without being married doubled again. In fact, the single-parent household, once unusual, has replaced the "traditional" family as the typical family in the States. If we can judge from history, however, this will probably change again in the twenty-first century. The Changes of the American Family
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