DataBase System Mathematical relations E Formally, given sets D,, D2,.. Dn, a relation r is a subset of Cartesian product笛卡尔积)D1×D2×…,×Dn Thus a relation(关系) is a set of n-tuples(a1,a2,…,an) where a;∈D H EXample: customer-name=Jones, Smith, Curry, Lindsay customer-Street=(Main, North, Park) customer-city =( Ha arrison. Rye, Pittsfield) Then r=(ones, Main, Harrison) (Smith, North, rye) (Curry, North, Rye) Lindsay, Park, Pittsfield)) is a relation over customer-name x customer-street x customer-citi Haichang Gao, Software School, Xidian UniversityDataBase System Haichang Gao , Software School , Xidian University 5 Mathematical Relations  Formally, given sets D1 , D2 , …. Dn , a relation r is a subset of Cartesian product(笛卡尔积) D1 × D2 × … × Dn Thus a relation(关系) is a set of n-tuples (a1 , a2 , …, an ) where ai  Di  Example: if customer-name = {Jones, Smith, Curry, Lindsay} customer-street = {Main, North, Park} customer-city = {Harrison, Rye, Pittsfield} Then r = {(Jones, Main, Harrison), (Smith, North, Rye), (Curry, North, Rye), (Lindsay, Park, Pittsfield)} is a relation over customer-name × customer-street × customer-city