07 Computer Applications
M3B: Relational Databases and Working with SQL (1)
M3B: Relational Databases and Working with SQL (1)
Basic Concepts of a Relational Database
Softcopy of handouts can be found at http://www.clsmss.edu.hk/~whlau/07CA/
1.
Relational database model
a) Entity: An entity is something that has a distinct, separate existence, though it need not be a
material existence. It can be a person, a place, a device, an event or a concept.
b) Relation: Relation is a named two-dimensional table, containing a set of named attribute
(columns) and an arbitrary number of unnamed tuple (rows).
c) Attribute: An attribute is a named column of a relation. Each attribute in an relation must
have a different name.
d) Domain: Data domain is the set of possible values for a given attribute. It involves data
type, size (length), and the allowable values.
e) Each tuple (row) of a relation corresponds to a record that contains attribute values for a
single entity.
f) The order of tuples in a relation is not important.
Ref: http://en.wikipedia.org/wiki/Database
http://en.wikipedia.org/wiki/Relational_database
http://en.wikipedia.org/wiki/Entity
2.
Keys:
a) A key is a kind of constraint which requires that the object, or critical information about the
object, isn’t duplicated.
b) Candidate key: A candidate key is set of attributes such that at all times there are no two
distinct tuples with the same values for these attributes and there is no proper subset of the
attributes.
c) Compound key: A compound key (also called a composite key or concatenated key) is a
key that consists of 2 or more attributes.
d) Primary key: A primary key is a value that can be used to identify a unique row in a table.
It is always chosen from one of the candidate key in an relation.
e)
Note that in most cases, primary key should be immutable, meaning that its value should not
be changed during the course of normal operations of the database. This avoids the
problem of dangling references or orphan records created by other relations referring to a
tuple whose primary key has changed.
f) Foreign key: A foreign key (FK) is a reference to a key in another table. Usually a foreign
key in one table refers to the primary key (PK) of another table. This way references can be
made to link information together and it is an essential part of database normalization.
Ref: http://en.wikipedia.org/wiki/Primary_key
http://en.wikipedia.org/wiki/Compound_key
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07 Computer Applications
M3B: Relational Databases and Working with SQL (1)
http://en.wikipedia.org/wiki/Candidate_key
http://en.wikipedia.org/wiki/Foreign_key
3.
Table Relationships
a) One-to-one relationship: Example, student to locker. Each student is assigned a locker,
whilst each locker belongs to one student only.
b) One-to-many relationship: Example, department to employees. One department has
many employees, but each employee is assigned to one department only.
c) Many-to-many relationship: Example, customers to products. A customer may order
many different products, and each product may be ordered by different customers. An
intermediary table (e.g. order table) is needed to hold the relationship.
4.
Constraints:
a) Key constraint: All tuples (records) in a relation must be distinct. No two tuples can have
the same combination of values for all their attributes.
b) Entity integrity constraint: No primary key can be NULL.
c) Domain integrity constraint: Value of each attribute must be an atomic value from the
domain for that attribute. E.g. Age: integer, 17 is acceptable but 17.4 is not.
d) Referential integrity constraint: Referential integrity is consistency between related tables.
It is usually enforced by the combination of a primary key or candidate key and a foreign key.
i. The relational database management system (RDBMS) enforces referential integrity,
normally either by deleting the foreign key rows as well to maintain integrity, or by
returning an error and not performing the delete.
ii. For example, the department number in employee table is a foreign key referring to the
key field in the department table. Deleting the record of a department will break
referential integrity.
Ref: http://en.wikipedia.org/wiki/Referential_integrity
5.
Students should be able to identify these basic elements in examples taken from everyday
applications
6. Other references:
http://www.cbe.wwu.edu/ganz/mis421web/421chap06.ppt
http://www.cs.nthu.edu.tw/~yishin/Courses/CS6710/Handout/Session3%20-%20Relational%20Model.pdf
http://joung.im.ntu.edu.tw/teaching/hospital_DB/2002/tables.ppt (Chinese)
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