问题
单项选择题
In order to get names of schoolboys(SEX='M')whose age=22, from a relation S(SNO,SNAME,SEX,AGE),which of the following expressions is (are) correct Ⅰ．πsname(σsex='M'^age=22(S)) Ⅱ．σsex='M'^age=22(πsname(S))
A． Ⅰ only
B． Ⅱ only
C． Ⅰand Ⅱ
D． neither
答案
参考答案：A