数据库ch ［精品pdf］

lecta froms) d. selecta fromrasr1,rasr2,s wherer1.a=s.aandr2.a=s.candr1.b>r2.b 6.18 LetR=(A,B) andS=(A,C), and letr(R) ands(S) be relations. Using the special constantnull, write tuple-relational-calculus expressions equivalent to each of the following: a. r1s b. r 1s c. r1s Answer: a. {t| ?r∈R?s∈S(r[A]=s[A]∧t[A]=r[A]∧t[B]=r[B]∧t[C]= s[C])∨?s∈S(??r∈R(r[A]=s[A])∧t[A]=s[A]∧t[C]=s[C]∧t[B]= null)} b. {t| ?r∈R?s∈S(r[A]=s[A]∧t[A]=r[A]∧t[B]=r[B]∧t[C]= s[C])∨?r∈R(??s∈S(r[A]=s[A])∧t[A]=r[A]∧t[B]=r[B]∧t[C]= null)∨?s∈S(??r∈R(r[A]=s[A])∧t[A]=s[A]∧t[C]=s[C]∧t[B]= null)} c. {t| ?r∈R?s∈S(r[A]=s[A]∧t[A]=r[A]∧t[B]=r[B]∧t[C]= s[C])∨?r∈R(??s∈S(r[A]=s[A])∧t[A]=r[A]∧t[B]=r[B]∧t[C]= null)} 6.19 Give a tuple-relational-calculus expression to ?nd the maximumvalue in relationr(A). Answer:{<a>|<a>∈r∧?<b>∈R a>=b}