DataBase System 沙Key Let Kcr e K is a superkey(ig)of R if values for K are sufficient to identify a unique tuple of each possible relation r(R) custoer id customer name Customer street customer cit 192-83-7465 Johnson 12 Alma St Palo alto 677-89-9011 Hayes 3 Main st Harrison 182-73-609 Turner 123 Putnam Ave Stamford 321-12-3123 ones 100 Main St Harrison 336-66-9999 Lindsay 175 Park Ave Pittsfield 019-28-3746 Smith 72 North St Rve (a) The customer table E Example: customer id, customer name) and customer id are both superkeys of Customer a by possible r” ve mean a relation r that could exist in the enterprise we are modeling Haichang Gao, Software School, Xidian University 11DataBase System Haichang Gao , Software School , Xidian University 11 Key  Let K  R  K is a superkey(超码) of R if values for K are sufficient to identify a unique tuple of each possible relation r(R).  Example: {customer_id, customer_name} and {customer_id} are both superkeys of Customer.  by “possible r ” we mean a relation r that could exist in the enterprise we are modeling