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Relational Calculus

. Relational Calculus. . R&G, Chapter 4. We will occasionally use this arrow notation unless there is danger of no confusion. Ronald Graham Elements of Ramsey Theory. Relational Calculus. Query has the form: { T | p ( T )} p(T) is a formula containing T

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Relational Calculus

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  1. Relational Calculus  R&G, Chapter 4 We will occasionally use this arrow notation unless there is danger of no confusion. Ronald Graham Elements of Ramsey Theory

  2. Relational Calculus • Query has the form: {T | p(T)} • p(T)is a formula containing T • Answer = tuples T for which p(T)= true.

  3. Formulae • Atomic formulae: R  Relation R.aopS.b R.aopconstant … op is one of • Aformula can be: • an atomic formula

  4. Free and Bound Variables • Quantifiers:  and  • Use of or bindsX. • A variable that is not bound is free. • Recall our definition of a query: • {T | p(T)} • Important restriction: • Tmust be the only free variable in p(T). • all other variables must be bound using a quantifier.

  5. Simple Queries {S |S Sailors S.rating > 7} • Find all sailors with rating above 7 • Find names and ages of sailors with rating above 7. • Note: S is a variable of 2 fields (i.e. S is a projection of Sailors) {S | S1 Sailors(S1.rating > 7 S.sname = S1.sname S.age = S1.age)}

  6. Joins Find sailors rated > 7 who’ve reserved boat #103 { S | SSailors  S.rating > 7  R(RReserves  R.sid = S.sid  R.bid = 103) }

  7. Joins (continued) Find sailors rated > 7 who’ve reserved a red boat • This may look cumbersome, but it’s not so different from SQL! { S | SSailors  S.rating > 7  R(RReserves  R.sid = S.sid  B(BBoats  B.bid = R.bid  B.color = ‘red’)) }

  8. Universal Quantification Find sailors who’ve reserved all boats { S | SSailors  BBoats (RReserves (S.sid = R.sid B.bid = R.bid)) }

  9. A trickier example… Find sailors who’ve reserved all Red boats { S | SSailors  B  Boats ( B.color = ‘red’  R(RReserves  S.sid = R.sid B.bid = R.bid)) } Alternatively… { S | SSailors  B  Boats ( B.color  ‘red’  R(RReserves  S.sid = R.sid B.bid = R.bid)) }

  10. a  b is the same as a  b b T F T F T a T T F

  11. A Remark: Unsafe Queries • syntactically correct calculus queries that have an infinite number of answers! Unsafe queries. • e.g., • Solution???? Don’t do that!

  12. Expressive Power • Expressive Power (Theorem due to Codd): • Every query that can be expressed in relational algebra can be expressed as a safe query in relational calculus; the converse is also true. • Relational Completeness: Query language (e.g., SQL) can express every query that is expressible in relational algebra/calculus. (actually, SQL is more powerful, as we will see…)

  13. Summary • Formal query languages — simple and powerful. • Relational algebra is operational • used as internal representation for query evaluation plans. • Relational calculus is “declarative” • query = “what you want”, not “how to compute it” • Same expressive power --> relational completeness. • Several ways of expressing a given query • a queryoptimizer should choose the most efficient version.

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