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CHAPTER 1 RELATIONAL DATABASE SYSTEMS AND ORACLE
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3. Each relationship is transformed into a foreign key (FK) constraint at the crow s foot side. 4. 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.
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Table Descriptions
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Tables 1-1 through 1-7 describe the structures of the case tables. Table 1-1. The EMPLOYEES Table
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EMPNO ENAME INIT JOB MGR BDATE MSAL COMM DEPTNO
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Description
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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
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Table 1-2. The DEPARTMENTS Table
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DEPTNO DNAME LOCATION MGR
Description
Unique department number Department name Department location (city) Employee number of the manager of the department
Table 1-3. The SALGRADES Table
Column
GRADE LOWERLIMIT UPPERLIMIT BONUS
Description
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
CHAPTER 1 RELATIONAL DATABASE SYSTEMS AND ORACLE
Table 1-4. The COURSES Table
Column
CODE DESCRIPTION CATEGORY DURATION
Description
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
Table 1-5. The OFFERINGS Table
Column
COURSE BEGINDATE TRAINER LOCATION
Description
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
Table 1-6. The REGISTRATIONS Table
Column
ATTENDEE COURSE BEGINDATE EVALUATION
Description
Employee number of the course attendee Course code Start date of the course offering Evaluation of the course by the attendee (positive integer on the scale 1 5)
PK, FK1 PK, FK2 PK, FK2
Table 1-7. The HISTORY Table
Column
EMPNO BEGINYEAR BEGINDATE ENDDATE DEPTNO MSAL COMMENTS
Description
Employee number Year component (4 digits) of BEGINDATE Begin date of the time interval End date of the time interval The number of the department worked for during the interval Monthly salary during the interval Allows for free text style comments
PK, FK1 PK FK2
CHAPTER 1 RELATIONAL DATABASE SYSTEMS AND ORACLE
In the description of the EMPLOYEES table, the COMM column deserves some special attention. This commission attribute is relevant only for sales representatives, and therefore contains structurally missing information (for all other employees). We could have created a separate SALESREPS table (with two columns: EMPNO and COMM) to avoid this problem, but for the purpose of this book, the table structure is kept simple. The structure of the DEPARTMENTS table is straightforward. Note the two foreign key constraints between this table and the EMPLOYEES table: an employee can work for a department or be the manager of a department. Note also that we don t insist that the manager of a department actually works for that department, and it is not forbidden for any employee to manage more than one department. The salary grades in the SALGRADES table do not overlap, although in salary systems in the real world, most grades are overlapping. In this table, simplicity rules. This way, every salary always falls into exactly one grade. Moreover, the actual monetary unit (currency) for salaries, commission, and bonuses is left undefined. The optional tax-free bonus is paid monthly, just like the regular monthly salaries. In the COURSES table, three CATEGORY values are allowed: GEN (general), for introductory courses BLD (build), for building applications DSG (design), for system analysis and design This means that these three values are the only values allowed for the CATEGORY column; this is an example of an attribute constraint. This would also have been an opportunity to design an additional entity (and thus another relational table) to implement course types. In that case, the CATEGORY column would have become a foreign key to this additional table. But again, simplicity was the main goal for this set of case tables. In all database systems, you need procedures to describe how to handle historical data in an information system. This is a very important and, in practice, far from trivial component of system design. In our case tables, it is particularly interesting to consider course offerings and course registrations in this respect. If a scheduled course offering is canceled at some point in time (for example, due to lack of registrations), the course offering is not removed from the OFFERINGS table, for statistical/historical reasons. Therefore, it is possible that the TRAINER and/or LOCATION columns are left empty; these two attributes are (of course) relevant only as soon as a scheduled course is going to happen. By the way, this brings up the valid question whether scheduled course offerings and real course offerings might be two different entities. Again, an opportunity to end up with more tables; and again, simplicity was the main goal here. Course registrations are considered synonymous with course attendance in our example database. This becomes obvious from the EVALUATION column in the REGISTRATIONS table, where the attendee s appreciation of the course is stored at the end of the course, expressed on a scale from 1 to 5; the meaning of these numbers ranges from bad (1) to excellent (5). In case a registration is canceled before a course takes place, we remove the corresponding row from the REGISTRATIONS table. In other words, if the BEGINDATE value of a course registration falls in the past, this means (by definition) that the corresponding course offering took place and was attended.
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