设有关系R=(A，B，C) ，与SQL语句select distinct A from R where B=17等价的关系代数表达式是________。
A．пA(σB=17()
B．σB=17(пA()
C．σB=17(пA,C()
D．пA,C(σB=17()
参考答案：A
解析：
[评析] 投影：对R的投影操作，实际上是从R中选择出若干属性列组成新的关系，记做：пA(R)＝{t[A]|t∈R}， A为R的属性列。
选择：又称为限制，它是关系R中满足给定条件的诸元组，记做：σF(R)＝{t|t∈R∧F(t)＝'真'}，其中F表示选择条件。
对于选项A，先从关系R中选择B=17的元组，然后输出A列，它与SQL语句等价，而对于选项B，先对R进行投影，形成一个只有属性A的关系，在该关系中无法执行σB＝17，运算，它不能与SQL语句等价。
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