The university database stores details about university students, courses, the semester a student took a particular course (and his mark and grade if he completed it), and what degree program each student is enrolled in. The database is a long way from one that’d be suitable for a large tertiary institution, but it does illustrate relationships that are interesting to query, and it’s easy to relate to when you’re learning SQL. We explain the requirements next and discuss their shortcomings at the end of this section.
Consider the following requirements list:
The university offers one or more programs.
A program is made up of one or more courses.
A student must enroll in a program.
A student takes the courses that are part of her program.
A program has a name, a program identifier, the total credit points required to graduate, and the year it commenced.
A course has a name, a course identifier, a credit point value, and the year it commenced.
Students have one or more given names, a surname, a student identifier, a date of birth, and the year they first enrolled. We can treat all given names as a single object—for example, “John Paul.”
When a student takes a course, the year and semester he attempted it are recorded. When he finishes the course, a grade (such as A or B) and a mark (such as 60 percent) are recorded.
Each course in a program is sequenced into a year (for example, year 1) and a semester (for example, semester 1).
The ER diagram derived from our requirements is shown in Figure . Although it is compact, the diagram uses some advanced features, including relationships that have attributes and two many-to-many relationships.
In our design:
Student is a strong entity, with an identifier, student_id, created to be the primary key used to distinguish between students (remember, we could have several students with the same name).
Program is a strong entity, with the identifier program_id as the primary key used to distinguish between programs.
Each student must be enrolled in a program, so the Student entity participates totally in the many-to-one EnrollsIn relationship with Program. A program can exist without having any enrolled students, so it participates partially in this relationship.
A Course has meaning only in the context of a Program, so it’s a weak entity, with course_id as a weak key. This means that a Course is uniquely identified using its course_id and the program_id of its owning program.
As a weak entity, Course participates totally in the many-to-one identifying relationship with its owning Program. This relationship has Year and Semester attributes that identify its sequence position.
Student and Course are related through the many-to-many Attempts relationships; a course can exist without a student, and a student can be enrolled without attempting any courses, so the participation is not total.
When a student attempts a course, there are attributes to capture the Year and Semester, and the Mark and Grade.
Source: oreilly
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