create barcode using vb.net The ERM Diagram of the Case in Java

Paint Data Matrix ECC200 in Java The ERM Diagram of the Case

We start with an ERM diagram depicting the logical design of our case, which means that it does not consider any physical (implementation-dependent) circumstances. A physical design is the next stage, when the choice is made to implement the case in an RDBMS environment, typically resulting in a table diagram or just a text file with the SQL statements to create the tables and their constraints.

Figure 1-5 shows the ERM diagram for the example used in this book. The ERM diagram shows seven entities, represented by their names in rounded-corner boxes. To maintain readability, most attributes are omitted in the diagram; only the key attributes are displayed.

We have several relationships between these entities. The ten crow s feet connectors in the diagram represent one-to-many relationships. Each relationship can be read in two directions. For example, the relationship between OFFERING and REGISTRATION should be interpreted as follows: Each registration is always for exactly one course offering. A course offering may have zero, one, or more registrations.

Course offerings without registrations are allowed. All one-to-many relationships in our case have this property, which is indicated in this type of diagram with a dotted line at the optional side of the relationship. Notice that we have two different relationships between EMPLOYEE and DEPARTMENT: each employee works for precisely one department, and each employee can be the manager of zero, one, or more departments. The EMPLOYEE entity also shows a recursive relationship (a relationship of an entity with itself) that implements the hierarchy within the company. Each entity in the ERM diagram has a unique identifier, allowing us to uniquely identify all occurrences of the corresponding entities. This may be a single attribute (for example, EMPNO for the EMPLOYEE entity) or a combination of attributes, optionally combined with relationships. Each attribute that is part of a unique identifier is preceded with a hash symbol (#); relationships that are part of a unique identifier are denoted with a small crossbar. For example, the unique identifier of the OFFERING entity consists of a combination of the BEGINDATE attribute and the relationship with the COURSE entity, and the unique identifier of the entity REGISTRATION consists of the two relationships to the EMPLOYEE and OFFERING entities. By the way, entities like REGISTRATION are often referred to as intersection entities; REGISTRATION effectively implements a many-to-many relationship between EMPLOYEE and OFFERING. An ERM diagram can be transformed into a relational table design with the following steps:

1. 2. 3. 4.

Each entity becomes a table. Each attribute becomes a column. Each relationship is transformed into a foreign key (FK) constraint at the crow s foot side. Each unique identifier becomes a component of the primary key (PK).

This mapping results in seven tables: EMPLOYEES, DEPARTMENTS, SALGRADES, COURSES, OFFERINGS, REGISTRATION, and HISTORY.

Number, unique for every employee Last name Initials (without punctuation) Job description of the employee The employee number of the employee s manager Date of birth Monthly salary (excluding bonus or commission) Commission component of the yearly salary (only relevant for sales reps) The number of the department for which the employee works

Unique department number Department name Department location (city) Employee number of the manager of the department

Unique salary grade number Lowest salary that belongs to the grade Highest salary that belongs to the grade Optional (tax-free) bonus on top of the monthly salary

PK ----

Course code; unique for each course Short description of the course contents Course type indicator (allowed values: GEN, BLD, and DSG) Course duration, expressed in days

PK ----

Course code Start date of the course offering Employee number of the employee teaching the course Location (city) where the course is offered

PK, FK PK FK --