R（A1，A2，A3）和S（A2，A3，A4）上进行
关系运算，与该关系表达式等价的是（ ）。
将该关系代数表达式转换为等价的SQL语句如下：
SELECT A1，A4 FROM R,S WHERE R.A2＜‘2017’（ ）
（注意：原选项有误，现已修正）
正确答案及解析
正确答案
DC
解析
暂无解析
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