, . , , , . , , . .
.
, . R ρS(A1, A2,An)(R). R S, A1, A2,.,An . , ρS(R).
5.5.
R S (.5.10) S .
ρS(X,C,D)(S) , S,. S .5.6, , .
³ R ³ S
A | B | C | D | |
³ R×ρS(X,C,D)(S)
. 5.10. R S S .
.
س ᒺ, , , , , . , 璺, - 璺. .
R∩S = R (R S)
- 璺 .
R S = σ(R´S)
璺 R S , R´S,
R.A1 = S.A1 AND R.A2 = S.A2 AND ××××AND R.An = S.An,
A1, A2, ××××An , R S. , ﳿ . L R, S, R. :
|
|
R S = πL(σC(R ´ S))
.
.5.9 . , , . ( ). , , . :
1. , ; Answer , ; ;
2. :=;
3. - , , .
5.6.
.5.9 . :
R(t, y, l, i, s, p):= σ length≧100 (Movie);
S(t, y, l, i, s, p):= σ studioName = Fox (Movie);
T(t, y, l, i, s, p):= R ∩ S;
Answer (title, year):=πt, y(T)
, , σ length>=100. , . . ˳ .