- A.见图A
- B.见图B
- C.见图C
- D.见图D
正确答案及解析
正确答案
解析
本题前一空分析：
A选项的关系代数表达式，错误之处在于选择的两个条件不应为“或”关系。
B选项的关系代数表达式，错误之处在于R与S仅做了笛卡尔积的操作，并没有把相同属性列做等值判断。应加上：2=4及3=5的选择条件才对。
C选项的关系代数表达式，与B选项有相同错误，同时投影列号还不正确。
后面一空，首先值得说明的是，选项中有一个共同的错误，即“S.A4＜’95’”应修改为“S.A4=’95’”。
选项中几个条件都是正确的，需要选择的，其实只是使用AND还是OR来进行连接。由于进行自然连接以及相关条件判断都是同时要成立的，所以必须都要用AND进行连接。
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