[大数据]CS 425 – Database Organization - 3 - Chapter 3 ：Formal Relational Query Language (2)

31、Tuple Relational Calculus

① A nonprocedural query language, where each query is of the form

{t | P (t ) }

② It is the set of all tuples t such that predicate P is true for t
③ is a tuple variable , t [A ] denotes the value of tuple t on attribute A
④ t ∈ r denotes that tuple t is in relation r
⑤ P is a formula similar to that of the predicate calculus

32、Predicate Calculus Formula

1. Set of attributes and constants
2. Set of comparison operators: (e.g., <, ≤, =,>, ≥)
3. Set of logical connectives: and (∧), or (v)? not (?)
4. Implication (?): x ? y, if x if true, then y is true
   x ? y ≡ ?x v y
5. Set of quantifiers:
   ?t ∈r (Q (t )) ≡ ”there exists” a tuple in t in relation r
   such that predicate Q (t ) is true
   ?t ∈r (Q (t )) ≡ Q is true “for all” tuples t in relation r

33、Example Queries

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