How should I write a relationship in the diamond?
Reading order: From left to right.
For example, an employee manages other employees.
ERD could represent superclass-subclass relationships.
"O" is overlapping, meaning there could be overlapping between each subclass entities. (A manager could also be a salesperson)
Besides, we can use "D" for disjoint subclasses.
A single line between Employee and its subclasses implies an optinal participation (an employee may not belong to any subclass), while a double line implies a mandatory participation (an employee must be in one or more subclasses).
An U-like symbol identify subclasses by pointing to them. (the entity the bottom of "U" points to is a subclass)
Schema represents abstract (strong and weak) entities and associated attributes and necessary relationships between entities.
A schema looks like this:
STUDENT( Stu_num, Stu_Lname, Stu_Fname, program_num@, age, gender)
(PK "Stu_num" should be underscored. I use italic instead)
Data dictionary contains metadata of attributes, with which you could understand the what, where, how, how much, who, when questions about data.
A good logical model should follow normal form of database.
Contains repeating values.
Each cell in the table contains only one value (so that the table can be represented in a relational database).
No partial dependencies.
Meaning: A non-primary-key attribute cannot be dependent on part of primary-key.
For example, in R(
A B C D), if
(A+B) -> C, D,
(B) -> D,
D is partially dependent on primary key (A+B), thus not satisfying 2NF.
No transitive dependencies.
Meaning: A non-primary-key attribute cannot determine non-primary-key attributes. (But a non-primary-key attribute can determine a primary-key attribute)
No non-CK dependencies.
Another way to say this: For all functional dependencies exist in the table, all their determinants are candidate keys.
Meaning: the second and third normal forms normalize non-prime attributes, and BCNF normalizes relationships between prime attributes.
For example (a model in 3NF but not in BCNF)
In R( A B C D), there are
(A+B) -> C, D;
(A+C) -> B, D;
(C) -> B.
With a non-primary-key attribute C determines primary-key attribute B, the model does not satisfy BCNF.
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