在关系R（A1，A2，A3）和S（A2，A3，A4）上进行πA1,A4（σA2＜＇2017＇∧A4=＇95＇（R?S））关系运算，与该关系表达式等价的是（ ）
- A.π1,4（σ2＜＇2017＇∨4=＇95＇（R?S））
- B.π1,6（σ2＜＇2017＇（R）×σ3=＇95＇（S））
- C.π1,4（σ2＜＇2017＇（R）×σ6=＇95＇（S））
- D.π1,6（σ2=4∧3=5（σ2＜＇2017＇（R））×σ3=＇95＇（S）））
正确答案及解析
正确答案
解析
选项A的“∨”应该改为“∧”；选项B与选项C不应该是笛卡尔积。自然连接与笛卡尔积的区别是：自然连接去掉了重复的属性列。
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