关系R、S如下图所示，关系代数表达式π3,4,5(σ1大于6(R×S)= （） ，对关系R、S进行自然连接后的属性列数和元组个数分别为（此空作答）。
- A.3和0
- B.3和2
- C.6和0
- D.6和2
正确答案及解析
正确答案
解析
R×S后的结果如表7.6所示。
π3,4,5(σ1大于6(R×S)=3、4、5列对应的是R.C、S.A、S.B，其中第一列小于第6列的值(即1大于6)是第一、第三元组。故第24题的答案选B。设A1，A2，...，An是R和S的公共属性，当且仅当R的元组r与S的元组s在1，A2，…，An上都一致时，元组s和r组合成为R和S自然连接的一个元组。由此可得，R和S的公共属性为A、B、C，但R和S在属性A、B、C上的值不等，故自然连接后，属性列数为3，元组个数为0。
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