tailieunhanh - Database Modeling & Design Fourth Edition- P24

Database Modeling & Design Fourth Edition- P24: Database technology has evolved rapidly in the three decades since the rise and eventual dominance of relational database systems. While many specialized database systems (object-oriented, spatial, multimedia, etc.) have found substantial user communities in the science and engineering fields, relational systems remain the dominant database technology for business enterprises. | 102 CHAPTER 5 Transforming the Conceptual Data Model to SQL create table individual indiv_id char 10 indiv_name char 20 indiv_addr char 20 primary key indiv_id create table employee emp_id char 10 job_title char 15 primary key emp_id foreign key emp_id references individual on delete cascade on update cascade create table customer cust_no char 10 cust_credit char 12 primary key cust_no foreign key cust_no references individual on delete cascade on update cascade Figure UML generalization and aggregation transformation rules and integrity rules are the same for both the disjoint and overlapping subtype generalizations. Another approach is to have a single table that includes all attributes from the supertype and subtypes the whole hierarchy in one table with nulls used when necessary. A third possibility is one table for each subtype pushing down the common attributes into the specific subtypes. There are advantages and disadvantages to each of these three approaches. Several software tools now support all three options Fowler 2003 Ambler 2003 . Database practitioners often add a discriminator to the supertype when they implement generalization. The discriminator is an attribute that has a separate value for each subtype and indicates which subtype to use to get further information. This approach works up to a point. However there are situations requiring multiple levels of supertypes and subtypes where more than one discriminator may be required. The transformation of an aggregation abstraction also produces a separate table for the supertype entity and each subtype entity. However Transformation Steps 103 there are no common attributes and no integrity constraints to maintain. The main function of aggregation is to provide an abstraction to aid the view integration process. In UML aggregation is a composition relationship not a type relationship which corresponds to a weak entity Muller 1999 . Multiple Relationships Multiple relationships among n .