A survey from the Center for Science in the Public Interest (CSPI), a Washington-based consumer group, found that the vending machines in middle and high schools are filled with candy, cookies, chips, soft drinks and other sugary beverages, and very few fruits or healthy snacks.
This latest news may confirm the obvious to some, but it comes at a time when about 20% to 30% of children are overweight or at risk of becoming so. Some schools and entire school districts have revamped what's sold in vending machines in an effort to offer foods that are considered healthier.
What are the vending machines seldom filled with?
A.soft drinks
B.sugary beverages
C.fruits
D.candy
第1题
A.πA,B(σC≠’C56’(R⋈S))
B.πA,B(R ⋈ S) C≠’C56′
C.R-πA,B(σC= ‘C56’(R⋈S))
D.R-πA,B(σC≠’C56’(R⋈S))
第2题
设有关系R(A,B,C)和S(C,D)。与SQL语句SELECT A,B,D FROM R, S WHERE R.C=S.C等价的关系代数表达式是()。
A.OR.C=SC(ЛA,B,D(R×S))
B.ЛA,B,D(σR.C=SC(R×S))
C.σR.C=SC((ЛA,BR)×(ЛDS))
D.σR.C=SC(ЛD(ЛA,BR)×S))
第3题
设有关系R(A,B,C)和S(C,D)。与SQL语句SELECT A,B,D FROM R, S WHERE R.C=S.C等价的关系代数表达式是()。
A.σR.C=S.C(ЛA,B,D(R×S))
B.ЛA,B,D(σR.C=S.C(R×S))
C.σR.C=S.C((ЛA,BR)×(ЛDS))
D.σR.C=S.C(ЛD(ЛA,BR)×S))
第4题
A.SELECT*FROM R,SWHERE R.C=S.C
B.SELECT A,B,D FROM R,S WHERE R.C=S.C
C.SELECT A,B,D FROM R,S WHERE R=S
D. SELECT A,B FROM R WHERE (SELECT D FROM S WHERE R.C= S.C)
第5题
A.πA,D(σR.C=S.C(R×S))
B.σR.C=S.C(πA,D(R×S))
C.σR.C=S.C(πD(πAR)×S))
D.σR.C=S.C(πAR)×(πDS))
第6题
设有关系R(A,B,C)和S(C,D)。与SQL语句Select A,B,D From R,S Where R.C=S.C等价的关系代数表达式是
A.σR.C=S.C(πA,B,D(R×S))
B.πA.B.D(σR.C =S.C(R×S))
C.σR.C=S.C((πA,BR)×(πDS))
D.σR.C=S.C(πD((πA,BR)×S))
第7题
设有关系R(A,B,C) 和S(C,D) 。与SQL语句select A,B,D from R,S where R.C=S.C等价的关系代数表达式是
A.σ[R.C=S.C](π[A,B,D](R×S))
B.π[A,B,D](σ[R,C=S.C](R×S))
C.σ[R.C=S.C]((π[A,B]R)×(n[D]S))
D.σ[R,C=S.C](π[(π[A,B]R)×S)
第8题
设有关系R(A,B,C)和S(C,D):与SQL语句SELECTA,B,DFROMR,S WHERER,c=S.c等价的关系代数表达式是
A.R,C=S.C(A,B,D(R×S))
B.AB.D(R.C=S.C(R×S))
C.R.C=S.C((A.BR)×(DS))
D.R.C=S,C(D((A.BR)×S))
第9题
设有关系R(A,B,C)和S(C,D)。与关系代数表达式πA,B,D(σR.C=S.C(R×S)等价的SQL语句是
A.SELECT*FROM R,S WHERE R.C=S.C
B.SELECT A,B,D FROM R,S WHERE R.C=S.C
C.SELE4T 1,2,8 FROM R,S WHERE R=S
D.SELECT A,B FROM R WHERE(SELECT D FROM S WHERE R.C=S.
第10题
设有关系R(A,B,C)和S(C,D),与关系代数表达式ΠA,B,D(σR,C=S.C(R×S))等价的SQL语句是______。
A.SELECT*FROMR,S WHERE R.C=S.C
B.SELECT A,B,D FROM R,S WHERE R.C=S.C
C.SELECT A,B,D FROM R,S WHERE R=S
D.SELECT A,B FROM R WHERE(SELECT D FROM S WHERE R.C=S.
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