2-rel-algebra - RelationalModel&Algebra...

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Zachary G. Ives University of Pennsylvania January 13, 2011 Some slide content courtesy of Susan Davidson & Raghu Ramakrishnan
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2 Recall Our Initial Discussion… There are a variety of ways of representing  data, each with trade-offs Free text [often need a human] Shapes/points in space “Objects” with “properties” In general, our emphasis will be on the last item … though there are spatial databases, OO  databases, text databases, and the like…
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3 The Relational Data Model (1970) Lessons from the Codd paper Let’s separate physical implementation from logical Model the data  independently  from how it will be used (accessed,  printed, etc.) Describe the data  minimally  and  mathematically A relation describes an association between data items –  tuples  with  attributes We generally think of tables and rows, but that’s somewhat  imprecise Use standard mathematical (logical) operations over the data –  these are the  relational algebra  or  relational calculus How does this model relate to objects, properties?  What are its  abilities and limitations?
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4 Why Did It Take So Many Years to  Implement Relational Databases? Codd’s original work:  1969-70 Earliest relational database research:  ~1976 Oracle “2.0”: 1979 Why the gap? 1. “You could do the same thing in other ways” 2. “Nobody wants to write math formulas” 3. “Why would I turn my data into tables?” 4. “It won’t perform well” What do you think?
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5 Getting More Concrete: Building a Database and Application 1. Start with a conceptual model “On paper” using certain techniques we’ll discuss next week We ignore low-level details – focus on logical representation 2. schema Design and codify (in SQL) the relations/tables Do  physical  layout – indexes, etc. 3. Import the data 4. Write applications using DBMS and other tools Many of the hard problems are taken care of by other people  (DBMS, API writers, library authors, web server, etc.)
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6 Conceptual Design for  CIS Student Course Survey STUDENT COURSE Takes name sid cid name PROFESSOR Teaches semester fid name exp-grade “Who’s taking what, and what grade do they expect?” This design is independent of the final form of the report!
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7 Example Schema Our focus now:  relational  schema – set of  tables Can have other kinds of  schemas – XML, object,  sid name
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2-rel-algebra - RelationalModel&Algebra...

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