Discussion Focus Efficient data management typically requires the use of computer database. A database is a shared, integrated computer structure that stores a collection of: End-user data that is the raw facts of interest to the end user. Metadata, or data about data, through which the end-user data are integrated and managed. Metadata describes the data characteristics and the set of relationships that links the data found within the database. For example the metadata component stores information such as the name of each data element, the type of values (numeric, dates, or text) stored to each data element, and whether the data element can be left empty. A database management system (DBMS) is a collection of programs that manage the database structure and controls access to the data stored in the database. The DBMS serves as the intermediary (mediator) between the user and the database. The database structure itself is stored as a collection of files, and the only way to access the data in those files is through the DBMS. Figure 1 Figure 1 emphasizes the point that the DBMS presents the end user (or application program) with a single integrated view of the data in the database. DBMS advantages: Share data among multiple applications and users Many different users views of data into a single all-compassing data repository Improved data access Minimized data inconsistency - The probability of data inconsistency is greatly reduced in a properly designed database. Database design focuses on how the database structure will be used to store and manage end-user data. Data modeling is the first step in designing a database, refers to the process of creating a specific data model for a determined problem domain. A data model is relatively simple representation, usually graphical, of more complex real-world data structures. Data modeling is iterative, progressive process. 1
Data Model Basic Building Blocks The basic building blocks of all data models are entities, attributes, relationships, and constraints. An entity is a person, place, or event about which data will be collected and stored. An attribute is a characteristic of an entity. For example, a STUDENT entity would be described by attributes such as student first name, student last name, faculty number, student address. A relationship describes an association among entities. For example, relation exists between teachers and classes that can be described as follows: a teacher can teach many classes, but a class can be taught by only one teacher. Data models use three types of relationships: one-to-one, oneto-many, and many-to-many. Usually the shortened notations are used: 1:1 or 1..1, 1:M or 1..*, M:N or *..* (may be seen as M:M or M..M) respectively. The following examples illustrate the distinctions among them: One-to-one (1:1 or 1..1) relationship. A professor chairs one department, and a department can be chaired by only one professor. Therefore the relationship PROFESSOR chairs DEPARTAMENT is labeled 1:1. One-to-many (1:M or 1..*) relationship: A painter creates many different paintings, but each painting is painted by only one painter. Thus the painter (the one ) is related to the paintings (the many ). Therefore the relationship PAINTER paints PAINTING is labeled 1:M. Similarly, a customer (the one ) may generate many invoices, but each invoice (the many ) is generated by only a single customer. The CUSTOMER generates INVOICES relationship would also be labeled one-to-many. Many-to-many (M:N or *..*) relationship: An employee may learn many job skills, and each job skill can may be learnt by many employees. The relationship EMPLOYEE learns SKILL is labeled as M:M. Similarly a student can take many classes and each class can be taken by many students. Again M:M relationship STUDENT takes CLASS A constraint is a restriction placed on the data. For example: An employee minimum salary must be 350lv A student s mark must be between 2 and 6. Each class must have one and only one teacher Business rules A business rule is a brief, precise, and unambiguous description of a policy, procedure, or a principle within a specific organization. For example, the business rule a teacher can teach many classes. From this business rule you could decide that: Teacher and class are objects of interest for the environment and should be represented by their respective entities. There is a teach relationship between teacher and class But this is not enough to identify the relationship properly. To do that you have to consider that relationships are bidirectional, that is they go both ways. For example, the business rules a teacher can teach many classes is complemented (completed) by the business rule a class can be taught by one and only one teacher. In this case the relationship is one-to-many (1:M). Teacher is the 1 side, and class is the many side. Relational model The characteristics of a relational table are as follows: A table is perceived as a two dimensional structure composed of intersecting rows and columns. 2
Each table row (tuple) represents a single entity occurrence within the entity set. Each table column represents an attribute (also called a field) and each column has a distinct name. Each intersection of row and column represents a single data value. All values in a column must conform to the same data format. Each column has a specific range of values known as the attribute domain. The order of the rows and columns is immaterial to the DBMS. Each table must have an attribute or a combination of attributes that uniquely identifies each row - primary key (PK) For example, the COURSE table in fig.2 might contain a professor s id that is also contained in PROFESSOR table. (Figure 2). The tables are linked through the common attribute PROF_ID. Figure 2 A foreign key is the primary key of one table that has been placed into another to create a common attribute. The relationship type (1:1, 1:M or M:M) is often shown in a relational schema, an example of which is shown in Figure 3. Figure 3 3
A relational diagram is a representation of relational database s entities, the attributes within those entities and the relationships between those entities. The entity relationship model The entity relationship (ER) model or ERM has become a widely accepted standard for data modeling. ERM models are normally represented in an entity relationship diagram (ERD), which uses graphical representations to model database components. A relational diagram is a visual representation of the relational database s entities, the attributes within those entities, and the relationships between those entities. There are different notations for graphical depiction of the main ER model components. We will use Crow s Foot Notation.(Figure 4) Figure 4 The ER model is based on the following components (entities and relationships): Entity - represented in the ERD by a rectangle. The entity name is generally written in capital letters and in singular form: PAINTER, STUEDENT, and COURSE. When applying the ERD to the relational model, an entity is mapped to a relational table. (Figure 5). Relationships. Relationships describe associations among data. The name Crow s foot is derived from the three-pronged symbol used to represent the many side of the relationship. The 1 side is represented by short line segment. Figure 6 Entity Relationship Diagram Relational table Figure 5 4
(1:M): a PROFESSOR may teach many COURSEs; a COURSE is taught by only one PROFESSOR (M:M): а STUDENT can take many CLASSEs; a CLASS can be taken by many STUDENTs (1:1): a PROFESSOR can chair only one DEPARTMENT; a DEPARTMENT is chaired by one PROFESSOR Figure 6 The M:N Relationship A many-to-many relationship is not supported directly in the relational environment However, M:N relationships can be implemented by creating a new entity in 1:M relationship with the original entities. A typical example is the relationship between STUDENT and CLASS, because each CLASS can have many students, and each STUDENT may take many CLASSes. The ERM is shown in Figure 7. Figure 7 Wrong implementation of M:N relationship is shown in Figure 8. 5
Figure 8 To solve the problems inherited in the M:N relationships we create a composite entity (bridge entity, associative entity) Because such a table is used to link the tables originally in M:N relationship, the bridge entity includes - as foreign keys at least the primary keys of the tables that are to be linked. To define the primary key for the new table there are two options: use the combination of those foreign keys or create a new one. Back to STUDENT CLASS example, a solution could be creating a composite ENROLL table shown in Figure 9 to link the tables STUDENT and CLASS. Here the primary key is combination of its foreign keys CLASS_ID and STUD_ID. 6
Figure 9 The revised relationship is shown in Figure 10 Figure 10 7
Problems: 1. Use Figure P1 as your guide. The DealCo relational diagram shows the initial entities and attributes for the DealCo stores, located in two regions of the country. Figure P1 The DealCo relational diagram a. Identify each relationship type and write all of the business rules. b. Create the basic Crow s Foot ERD for DealCo. 2. Use Figure P2 as your guide. The Tiny College relational diagram shows the initial entities and attributes for Tiny College. Figure P2. The Tiny College Relational Diagram a. Identify each relationship type and write all of the business rules. b. Create the basic Crow s Foot ERD for Tiny College. 3. Create a Crow s Foot ERD to include the following business rules for the ProdCo company: a. Each sales representative writes many invoices. b. Each invoice is written by one sales representative. c. Each sales representative is assigned to one department. d. Each department has many sales representatives. e. Each customer can generate many invoices. f. Each invoice is generated by one customer. 4. Write the business rules that are reflected in the ERD shown in Figure P3. (Note that the ERD reflects some simplifying assumptions. For example, each book is written by only one author. Also, remember that the ERD is always read from the 1 to the M side, regardless of the orientation of the ERD components.) 8
Figure P3 5. Create a Crow s Foot ERD for each of the following descriptions. (Note: The word many merely mean more than one in the database modeling environment.) a. Each of the MegaCo Corporation s divisions is composed of many departments. Each of those departments has many employees assigned to it, but each employee works for only one department. Each department is managed by one employee, and each of those managers can manage only one department at a time. b. During some period of time, a customer can rent many videotapes from the BigVid store. Each of the BigVid s videotapes can be rented to many customers during that period of time. c. An airliner can be assigned to fly many flights, but each flight is flown by only one airliner. 6. Suppose you have the ERM shown in Figure P4 How would you convert this model into an ERM that displays only 1:M relationships. Draw the revise diagram. 7. Use the database shown in Figure P5 to answer the following questions: a. Identify the primary keys. b. Identify the foreign keys 9
Figure P5 c. Create the ERM CASE: TinyVideo is a small movie rental company with a single store. TinyVideo needs a database system to track the rental of movies to its members. TinyVideo can own several copies (VIDEO) of each movie (MOVIE). For example, the store may have 10 copies of the movie Twist in the Wind. Twist in the Wind would be one MOVIE and each copy would be a VIDEO. A rental transaction (RENTAL) involves one or more videos being rented to a member (MEMBERSHIP). A video can be rented many times over its lifetime, therefore, there is a M:N relationship between RENTAL and VIDEO. Create the ERM. 10