Database Systems, Relational Algebra & Calculus (2000).pdf

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Chapter 3
Relational algebra
and calculus
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• Operations on databases:
– queries: "read" data from the database
– updates: change the content of the database
• Both can be modeled as functions from databases to databases
• Foundations can be studied with reference to query languages:
– relational algebra, a "procedural" language
– relational calculus, a "declarative" language
–(briefly) Datalog, a more powerful language
• Then, we will study SQL, a practical language (with declarative
and procedural features) for queries and updates
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• A collection of operators that
– are defined on relations
– produce relations as results
and therefore can bined to plex expressions
• Operators
– union, intersection, difference
– renaming
– selection
– projection
– join (natural join, cartesian product, theta join)
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• Relations are sets, so we can apply set operators
• However, we want the results to be relations (that is,
homogeneous sets of tuples)
• Therefore:
– it is meaningful to apply union, intersection, difference only
to pairs of relations defined over the same attributes
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Graduates
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