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【单选题】
有关系SC(S_ID,C_ID,AGE,SCORE),查找年龄大于22岁的学生的和分数,正确的关系代数表是( ) 。 i. πS_ID,SCORE (σ age>22 (SC) )iiσage>22 (πS_ID,SCORE (SC) ) iii. πS_ID,SCORE (σage>22 (πS_ID,SCORE,AGE (SC) ) )A.
ⅰ和 ⅱ
B.
只有ⅱ正确
C.
只有 ⅰ正确
D.
ⅰ和ⅲ正确
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