看关系R.S如下表所示，则关系R与S进行自然连接运算后的属性列数和元组个数分别为(作答此空)，关系代数表达式π15(σ2=5(R X S))与关系代数表达式（）等价。
- A.6和7
- B.4和4
- C.4和3
- D.3和4
正确答案及解析
正确答案
解析
自然连接是在等值连接的基础上去掉重复的属性列，而等值连接又要求R和S有相等的属性才能连接。自然连接的结果是6615、6151、6154。
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