CS 4400 Chapter Notes - Chapter 8: Railways Act 1921, Xu, Thx
24 Dec 2017
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Document Summary
Chapter 8: the relational algebra and relational calculus. Operations used to specify basic retrieval requests (queries) Result of an operation is a new relation which can be formed by one or more input relations. Relational algebra expression: sequence of relational algebra operations. Result is a relation representing result of a db query. Muhammad ibn musa al-khwarizmi from morocco wrote a book called al- jabr about arithmetic of variables. Called variables shay --> arabic for thing --> spanish xay --> x. Used to select a subset of the tuples from a relation based on a selection condition. Condition must be boolean expression on attributes of. Produces a relation s with the same schema/attributes as r. Select is commutative so selection can be applied in any order. Number of tuples is less than or equal to number of number of tuples in input relation. Keeps certain columns - attributes - from a relation and discards others.
