Question

数据库的题目，急急急，不会的不要答

Answer 1

RUS={(3,6,7),(2,5,7),(7,2,3),(3,4,5)}
R-S={(3,6,7),(2,5,7)}
RNS={(7,2,3)}
RXS={(3,6,7,3,4,5),(3,6,7,7,2,3),(2,5,7,3,4,5),(2,5,7,7,2,3),(7,2,3,3,4,5),(7,2,3,7,2,3)}

Answer 2

RUS
A B C
3 6 7
2 5 7
7 2 3
3 4 5

R-S
A B C
3 6 7
2 5 7

RnS
A B C
7 2 3

RXS
A B C A B C
3 6 7 3 4 5
3 6 7 7 2 3
2 5 7 3 4 5
2 5 7 7 2 3

Answer 3

RUS
select A,B,C from R
union
select A,B,C from S
R-S
select r.A,r.B,r.C from R r left join S s on r.A=s.A and r.B=s.B and r.C=s.C where s.C is null
RnS
select r.A,r.B,r.C from R r,S s where r.A=s.A and r.B=s.B and r.C=s.C
RxS
select r.A,r.B,r.C,c.A,c.B,c.C from R r,S s