DBS lecture 2

Relations and relational algebra

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A set of pairs

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Degree of a relation

How long the tuples are or how many columns the table has

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Cardinality of the relation

How many different tuples there are or how many different rows the table has

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{a,b,c} U {a,d,e} = {a,b,c,d,e}

In order to take a union of relations R and S, R and S must have the same number of columns and the corresponding columns must have the same domains

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{a,b,c,d} n {a,d,e,f} = {a,d}

Intersection is a relation R n S which contains tuples which are in both R and S

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π 2 (R) takes the 2nd column from table R

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