L03 - relational databases and logic

L03 - relational databases and logic - Relational Model...

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Unformatted text preview: Relational Model Basics Relational Model Basics Remember that an n-ary relation defined on D 1 , D 2 , , D n is just a subset of D 1 D 2 D n Codds observation was that a relation can be viewed as a table Does this represent the same relation? Employee Department Skill Jan accounting COBOL Steve net services Java Tom engineering C++ Employee Department Skill Tom engineering C++ Jan accounting COBOL Steve net services Java Set Theoretic Semantics Set Theoretic Semantics The data in the two tables is the same, but the order of the rows is different On a physical level, they are different, but at a logical level, when viewed as relations, they are the same the order of the rows (or tuples) in a relation is of no significance the order of the columns is of significance (think of the relation < defined on R R) What about multiple values? What about multiple values? Suppose, in our prior example, Steve, Jan, and Tom each were competent in more than one language? Employee Department Skill Jan accounting COBOL Steve net services Java, C++ Tom engineering C++, Java, Matlab We cant just put a list of languages and systems in the column for skill each entry in a column needs to be a skill, not a list of skills if we allowed a pair or a triple to be an element of a column, wed have to make pairs, triples, etc. part of the domain First Normal Form First Normal Form A database schema is in first normal form (1NF) if the following three conditions hold: domain values are atomic relations are sets the order of the tuples plays no role Employee Department Skill Steve net services Java Steve net services C++ Tom engineering C++ Tom engineering Java Tom engineering Matlab Jan accounting Cobol Fundamental Definitions (i) Fundamental Definitions (i) We have to distinguish between a relation and its description (or a database and its definition)...
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L03 - relational databases and logic - Relational Model...

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