数据库第二章 5节.ppt

DEFINITION 2.6.1 Compatible Tables Table R and S are compatible if they have the same headings,with attributes chosen from the same domains and with the same meanings. R S The Union, Intersection, and Difference Operations Only tables that are compatible can be involved in Union, Intersection, and differences. DEFINITION 2.6.2 Union, Intersection, and Difference Let table R and S are compatible, where Head( R) = Head( S) =A1…An. The union of R and S is the table R∪ S, with the same heading, consisting of all rows that are in R or in S or in both. Similarly, the Intersection of R and S is the table R ∩ S, consisting of those rows that are in both R and S . R S R∪S R∩S ? R S R∪S R S R∩S The Difference of R and S is the table R - S, consisting of all rows that appear in R but do not appear in S. R S R-S S - R? R S R - S Assignment and Alias During evaluation of relational algebra expression, it is sometimes useful to be able to save certain intermediate results. We next introduce a notation meant to add this capability to relational algebra. DEFINITION 2.6.3 Assignment and Alias Let R be a table and let Head( R) = A1…An. Assume that B1,…,Bn are n attributes such that Domain(Bi)= Domain(Ai) for all i, 1≤i≤n.We define a new table S, whose heading is Head(S) = B1…Bn, by writing the assignment S(B1,…,Bn):= R(A1,…,An). The content of the new table S is exactly the same as the content of the old table R. The symbol := used in this assignment is called the assignment operation. Alias S := R We refer to S as an alias of the table R. Note that the table R on the right can result from an e