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Unit 2: Relational Query Languages

20247m

Difference between Relational Algebra & Calculus.

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Relational Algebra vs Calculus:

Relational Algebra: Procedural language - specifies sequence of operations (select, project, join) to compute result. Operations on relations produce relations (closed). Example: pi_name(sigma_age>20(Student)). Implementable, defines how to get data.

Relational Calculus: Declarative/non-procedural - specifies what to retrieve using logic predicate, not how. Tuple calculus: {t | P(t)} where P is formula using quantifiers. Domain calculus similar. Example: {t.name | Student(t) AND t.age>20}. No order specified, optimizer decides.

Differences:

  • Algebra procedural, calculus declarative.
  • Algebra has operators; calculus has predicates, quantifiers (exists, forall).
  • Both relationally complete (equivalent expressive power for safe queries), but calculus allows non-safe infinite results unless domain restricted.
  • SQL borrows from both: SELECT-FROM-WHERE is calculus style but execution uses algebra.

Tuple vs Domain calculus variant distinction included.

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