TAILIEUCHUNG - Database Modeling & Design Fourth Edition- P47

Database Modeling & Design Fourth Edition- P47: 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. | Data Definition Language DDL 217 designate composite candidate keys that are not the primary key. This is particularly useful when transforming ternary relationships to SQL. Primary key the primary key is a set of one or more attributes that when taken collectively enables us to uniquely identify an entity or table. The set of attributes should not be reducible see Section . The designation primary key for an attribute implies that the attribute must be not null and unique but the SQL keywords NOT NULL and UNIQUE are redundant for any attribute that is part of a primary key and need not be specified in the create table command. Foreign key the referential integrity constraint specifies that a foreign key in a referencing table column must match an existing primary key in the referenced table. The references clause specifies the name of the referenced table. An attribute may be both a primary key and a foreign key particularly in relationship tables formed from many-to-many binary relationships or from n-ary relationships. Foreign key constraints are defined for deleting a row on the referenced table and for updating the primary key of the referenced table. The referential trigger actions for delete and update are similar on delete cascade the delete operation on the referenced table cascades to all matching foreign keys. on delete set null foreign keys are set to null when they match the primary key of a deleted row in the referenced table. Each foreign key must be able to accept null values for this operation to apply. on delete set default foreign keys are set to a default value when they match the primary key of the deleted row s in the reference table. Legal default values include a literal value user system user or no action. on update cascade the update operation on the primary key s in the referenced table cascades to all matching foreign keys. on update set null foreign keys are set to null when they match the old primary key value of an updated .

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