Relational Database Model

Brief history of the Relational Model

The relational model was first proposed by E. F. Codd in his seminal paper “A relational model of data for large shared data banks”. This paper is now generally accepted as a landmark in database systems, although a set-oriented model had been proposed previously. The relational model’s objectives were specified as follows:

· To allow a high degree of data independence. Application programs must not be affected by modifications to the internal data representation.

· To provide substantial grounds for dealing with data semantics, consistency, and redundancy problems.

· To enable the expansion of set-oriented data manipulation languages.

Although interest in the relational model came from several directions, the most significant research may be attributed to three projects with rather different perspective. These three projects where: Prototype relational DBMS System R, INGRES – Interactive Graphics Retrieval), Peterlee Relational Test Vehicle .

Relational Model

The relational model is based on the mathematical concept of a relation, which is physically represented as a table. Codd, used terminology taken from mathematics, principally set theory and predicate logic.

Relation is a table with columns and rows. Attribute is a named column of a relation. In relational model, relations are used to hold information about the objects to be represented in a database. Attributes can appear in any order and the relation will still be the same relation, and therefore convey the same meaning.


The relational model used the basic concept of a relation or table. The columns or fields in the table identify the attributes such as name, age, and so.

For example, the information on branch offices is represented by the Branch relation, with columns for attributes branchNo, street, city, and postcode.

Domains are an extremely powerful feature of the relational model. Every attribute in a relation is defined on a domain. Domains may be distinct for each attribute, or two or more attributes may be defined on the same domain.

Tuple is a row of a relation. The elements of a relation are the rows or tuples in the table. In the Branch relation, each row contains four values, one for each attribute. Tuples can appear in any order and the relation will still be the same relation. The structure of a relation, together with a specification of the domains and any other restrictions on possible values, is sometimes called its intension. The tuples are called the extension of a relation, which changes over time.

Degree of a relation is the number of attributes it contains. The Branch relation has four attributes or degree four. This means that each now of the table is a tuple containing four values. A relation with only one attribute would have degree one and be called a unary relation. A relation with two attributes is called binary, one with three attributes is called ternary, and after that the term n-ary is usually used.

Cardinality is the number of tuples it contains. The number of tuples is called the cardinality of the relation and this changes as tuples are added or deleted. The cardinality is a property of the extension of the relation and is determined from the particular instance of the relation at any given moment.

Relational database is a collection of normalized relations with distinct relation names. A relational database consists of relations that are appropriately structured. We refer to this appropriateness as normalization.

Formal Terms      Alternative 1       Alternative 2

Relation                  Table                          File

Tuple                        Row                       Record

Attribute                 Column                         Field

The terminology for the relational model can be quite confusing. A third set of terms is sometimes used: a relation may be referred to as a file, the tuples as records, and the attributes as fields.

Mathematical Relations

To understand the meaning of relation, we have to review some concepts from mathematics. Suppose that we have two sets, D1 and D2, where D1 = {2, 4} and D2 = {1, 3, 5}. The Cartesian product of these two sets, written D1xD2, is the set of all ordered pairs such that the first element is a member of D1, and the second element is a member of D2.

Relation Schema

One example of the tuples: {(B005, 22 Deer Rd, London, SW1 4EH)} Or more correctly: {(branchNo: B005, street: 22 Deer Rd, city: London, postcode: SW1 4EH)}.

Relational Database Schema

It is a set of relation schemas, each with a distinct name. If R1, R2, …,Rn are a set of relation schemas, then we can write the relational database schema, or simply relational schema, R, as: R = {R1, R2, …, Rn}.

Properties of Relations includes the relation has a name that is distinct from all other relation names in the relational schema. Each cell of the relation contains exactly one atomic (single) value. Each attribute has distinct name. The values of an attribute are all from the same domain. Each tuple is distinct; there is no duplicate tuple. The order of attributes has no significance. The order of tuples has no significance, theoretically.

Relational Keys

We need to be able to identify one or more attributes that uniquely identifies each tuple in relation. Superkey an attribute, or set of attributes, that uniquely identifies a tuple within a relation. Candidate key a superkey such that no proper subset is a superkey within the relation. A candidate key, K, for a relation R has two properties, Uniqueness and Irreducibility.

There may be several candidate keys for a relation. When a key consists of more than one attribute, we call it a composite key. Primary key – the candidate key that is selected to identify tuples uniquely within the relation. Foreign Key – an attribute, or set of attributes, within one relation that matches the candidate key of some relation.

Relational Integrity

Since every attribute has an associated domain, there are constraints that form restrictions on the set of values allowed for the attributes of relations. There are two rules in integrity for the relational model known as entity integrity and referential integrity.

Null represents a value for an attribute that is currently unknown or it is not applicable for this tuple. A null can be taken to mean the logical value ‘unknown’. It mean that a value is not applicable to a particular tuple, or it could merely mean that no value has yet been supplied.

Entity Integrity. In a base relation, no attribute of a primary key can be null. Referential Integrity. If a foreign key exists in a relation, either the foreign key value must match a candidate key value of some tuple in its home relation or the foreign key vale must be wholly null.

Views

A view is virtual or derived relation: a relation that does not necessarily exist in its own right, but may be dynamically derived from one or more base relations. Base Relation is a named relation corresponding to an entity in the conceptual schema, whose tuples are physically stored in the database.

Purpose of views includes: It provides a powerful and flexible security mechanism. It permits users to access data in a way that is customized to their needs. And it can simplify complex operations on the base relations.

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Comments 2 comments

Scott Levy 4 years ago

Great post. Here’s a tutorial that shows how you can easily build an online database-driven web application with a parent-child table relationship, without codinghttp://blog.caspio.com/web-database/creating-one-t...


Plagiarism Sucks 2 years ago

Plagiarized word for word from Chapter 4 of "Database Systems: A Practical Approach to Design, Implementation, and Management", by Thomas Connolly and Carolyn Begg. Editing it down doesn't make it any more OK, especially without giving credit to the authors while you have your picture and screen name essentially taking credit for it...

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