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【单选题】
已知关系R(A, B.C), 其主码是属性A。下列SQL语句中定义了一个唯一 辅索引的是A.
create unique index idx2 on R(A)
B.
create chustered index idx3 on RC)
C.
create unique index idx1 on RB)
D.
ceate chustered index idx4 on R(A)
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