Database Modeling & Design Fourth Edition- P20 ppt

82 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling ships and process-to-data relationships. The conceptual data modeling step (ER approach) involves the classification of entities and attributes first, then the identification of generalization hierarchies and other abstractions, and finally the definition of all relationships among entities. Relationships may be binary (the most common), ternary, and higher-level n-ary. Data modeling of individual requirements typically involves creating a different view for each end user’s requirements. Then the designer must integrate those views into a global schema, so that the entire database is pictured as an integrated whole. This helps to elimi- nate needless redundancy—such elimination is particularly important in logical design. Controlled redundancy can be created later, at the physi- cal design level, to enhance database performance. Finally, an entity cluster is a grouping of entities and their corresponding relationships into a higher-level abstract object. Clustering promotes the simplicity that is vital for fast end-user comprehension. In the next chapter we take the global schema produced from the conceptual data modeling and view integration steps, and we transform it into SQL tables. The SQL for- mat is the end product of logical design, which is still independent of any particular database management system. 4.7 Literature Summary Conceptual data modeling is defined in Tsichritzis and Lochovsky [1982], Brodie, Mylopoulos, and Schmidt [1984], Nijssen and Halpin [1989], Batini, Ceri, and Navathe [1992]. Discussion of the requirements data collection process can be found in Martin [1982], Teorey and Fry [1982], and Yao [1985]. View integration has progressed from a represen- tation tool [Smith and Smith, 1977] to heuristic algorithms [Batini, Len- zerini, and Navathe, 1986; Elmasri and Navathe, 2003]. These algorithms are typically interactive, allowing the database designer to make decisions based on suggested alternative integration actions. A variety of entity clustering models have been defined that provide a useful founda- tion for the clustering technique shown here [Feldman and Miller, 1986; Dittrich, Gotthard, and Lockemann, 1986; Teorey et al., 1989]. Teorey.book Page 82 Saturday, July 16, 2005 12:57 PM 83 5 Transforming the Conceptual Data Model to SQL his chapter focuses on the database life cycle step that is of particular interest when designing relational databases: transformation of the conceptual data model to candidate tables and their definition in SQL [step II(c)]. There is a natural evolution from the ER and UML data models to a relational schema. The evolution is so natural, in fact, that it sup- ports the contention that conceptual data modeling is an effective early step in relational database development. This contention has been proven to some extent by the widespread commercialization and use of software design tools that support not only conceptual data modeling but also the automatic conversion of these models to vendor-specific SQL table definitions and integrity constraints. In this chapter we assume the applications to be Online Transaction Processing (OLTP). Note that Online Analytical Processing (OLAP) applications are the subject of Chapter 8. 5.1 Transformation Rules and SQL Constructs Let’s first look at the ER and UML modeling constructs in detail to see how the rules about transforming the conceptual data model to SQL tables are defined and applied. Our example is drawn from the company personnel and project conceptual schemas illustrated in Figure 4.3 (see Chapter 4). T Teorey.book Page 83 Saturday, July 16, 2005 12:57 PM 84 CHAPTER 5 Transforming the Conceptual Data Model to SQL The basic transformations can be described in terms of the three types of tables they produce: • SQL table with the same information content as the original entity from which it is derived. This transformation always occurs for entities with binary relationships (associations) that are many-to- many, one-to-many on the “one” (parent) side, or one-to-one on either side; entities with binary recursive relationships that are many-to-many; and entities with any ternary or higher-degree relationship or a generalization hierarchy. • SQL table with the embedded foreign key of the parent entity. This transformation always occurs for entities with binary relationships that are one-to-many for the entity on the “many” (child) side, for one-to-one relationships for one of the entities, and for each entity with a binary recursive relationship that is one-to-one or one-to-many. This is one of the two most common ways design tools handle relationships, by prompting the user to define a foreign key in the child table that matches a primary key in the parent table. • SQL table derived from a relationship, containing the foreign keys of all the entities in the relationship. This transformation always occurs for relationships that are binary and many-to-many, relationships that are binary recursive and many-to-many, and all relationships that are of ternary or higher degree. This is the other most common way design tools handle relationships in the ER and UML models. A many-to-many relationship can only be defined in terms of a table that contains foreign keys that match the primary keys of the two associated entities. This new table may also contain attributes of the original relationship—for example, a relationship “enrolled-in” between two entities Student and Course might have the attributes “term” and “grade,” which are associated with a particular enrollment of a student in a particular course. The following rules apply to handling SQL null values in these transformations: • Nulls are allowed in an SQL table for foreign keys of associated (referenced) optional entities. • Nulls are not allowed in an SQL table for foreign keys of associated (referenced) mandatory entities. Teorey.book Page 84 Saturday, July 16, 2005 12:57 PM 5.1 Transformation Rules and SQL Constructs 85 • Nulls are not allowed for any key in an SQL table derived from a many-to-many relationship, because only complete row entries are meaningful in the table. Figures 5.1 through 5.8 show the SQL create table statements that can be derived from each type of ER or UML model construct. Note that table names are shown in boldface for readability. Note also that in each SQL table definition, the term “primary key” represents the key of the table that is to be used for indexing and searching for data. 5.1.1 Binary Relationships A one-to-one binary relationship between two entities is illustrated in Figure 5.1, parts a through c. Note that the UML equivalent binary asso- ciation is given in Figure 5.2, parts a through c. When both entities are mandatory (Figure 5.1a), each entity becomes a table, and the key of either entity can appear in the other entity’s table as a foreign key. One of the entities in an optional relationship (see Department in Figure 5.1b) should contain the foreign key of the other entity in its transformed table. Employee, the other entity in Figure 5.1b, could also contain a foreign key (dept_no) with nulls allowed, but this would require more storage space because of the much greater number of Employee entity instances than Department instances. When both entities are optional (Figure 5.1c), either entity can contain the embedded foreign key of the other entity, with nulls allowed in the foreign keys. The one-to-many relationship can be shown as either mandatory or optional on the “many” side, without affecting the transformation. On the “one” side it may be either mandatory (Figure 5.1d) or optional (Fig- ure 5.1e). In all cases the foreign key must appear on the “many” side, which represents the child entity, with nulls allowed for foreign keys only in the optional “one” case. Foreign key constraints are set accord- ing to the specific meaning of the relationship and may vary from one relationship to another. The many-to-many relationship, shown in Figure 5.1f as optional for both entities, requires a new table containing the primary keys of both entities. The same transformation applies to either the optional or mandatory case, including the fact that the not null clause must appear for the foreign keys in both cases. Note also that an optional entity means that the SQL table derived from it may have zero rows for that particular relationship. This does not affect “null” or “not null” in the table definition. Teorey.book Page 85 Saturday, July 16, 2005 12:57 PM @Spy 86 CHAPTER 5 Transforming the Conceptual Data Model to SQL Figure 5.1 ER model: one-to-one binary relationship between two entities (b) One-to-one, one entity optional, one mandatory (a) One-to-one, both entities mandatory (c) One-to-one, both entities optional Desktop Engineer Employee managed-by 1 1 has- allocated 1 1 Department Abbreviation Report 1 1 has-abbr Every department must have a manager, but an employee can be a manager of at most one department. Some desktop computers are allocated to engineers, but not necessarily to all engineers. Every report has one abbreviation, and every abbreviation represents exactly one report. create table (report_no integer, report_name varchar(256), primary key(report_no); report create table (abbr_no char(6), report_no integer not null unique, primary key (abbr_no), foreign key (report_no) references on delete cascade on update cascade); abbreviation report create table (dept_no integer, dept_name char(20), mgr_id char(10) not null unique, primary key (dept_no), foreign key (mgr_id) references on delete set default on update cascade); department employee create table (emp_id char(10), emp_name char(20), primary key (emp_id)); employee create table (emp_id char(10), desktop_no integer, primary key (emp_id)); engineer create table (desktop_no integer, emp_id char(10), primary key (desktop_no), foreign key (emp_id) references on delete set null on update cascade); desktop engineer Teorey.book Page 86 Saturday, July 16, 2005 12:57 PM . conceptual data modeling and view integration steps, and we transform it into SQL tables. The SQL for- mat is the end product of logical design, which is still independent of any particular database. Conceptual Data Model to SQL his chapter focuses on the database life cycle step that is of particular interest when designing relational databases: transformation of the conceptual data model. conceptual data modeling is an effective early step in relational database development. This contention has been proven to some extent by the widespread commercialization and use of software design tools