Tips & Tricks

Merits of Database Management Systems

Database image
Written by Victor Ikani

The advantages of Database Management Systems ( DMS) is laudable across sectors of the economy; especially in this jet age. Nigeria as a country is growing technologically, being her citizens are becoming technology merchants and Guru in most computer programs. We at Nolly Technologies are demonstrating this prowess in various programs such as Database Management. We train individual and corporate body on Database Management because is cruiser to organization effective management. We go beyond training; we also go charter by organizations to manage their database.
This article is not meant to tell you what we do regarding Database Management, rather to educate you on how to go about setting up your database and manage it all by yourself.
Read this carefully, try it over and over on your own… the example below is applicable to other information or data.
• Domain: a set of atomic values that an attribute can take
• Attribute: name of a column in a particular table (all data is stored in tables). Each attribute Ai must have a domain, dom(Ai).
• Relational Schema: The design of one table, containing the name of the table (i.e. the name of the relation), and the names of all the columns, or attributes.
Example: STUDENT( Name, SID, Age, GPA)
• Degree of a Relation: the number of attributes in the relation’s schema.
• Tuple, t, of R( A1, A2, A3, …, An): an ORDERED set of values, , where each vi is a value from dom( Ai).
• Relation Instance, r( R): a set of tuples; thus, r( R) = { t1, t2, t3, …, tm}
NOTES:
1. The tuples in an instance of a relation are not considered to be ordered __ putting the rows in a different sequence does not change the table.
2. Once the schema, R( A1, A2, A3, …, An) is defined, the values, vi, in each tuple, t, must be ordered as t =
Properties of relations
Properties of database relations are:
• relation name is distinct from all other relations
• each cell of relation contains exactly one atomic (single) value
• each attribute has a distinct name
• values of an attribute are all from the same domain
• order of attributes has no significance
• each tuple is distinct; there are no duplicate tuples
• Order of tuples has no significance, theoretically.
Relational keys
There are two kinds of keys in relations. The first are identifying keys: the primary key is the main concept, while two other keys – super key and candidate key – are related concepts. The second kind is the foreign key
Identity Keys
Super Keys
A super key is a set of attributes whose values can be used to uniquely identify a tuple within a relation. A relation may have more than one super key, but it always has at least one: the set of all attributes that make up the relation.
Candidate Keys
A candidate key is a super key that is minimal; that is, there is no proper subset that is itself a superkey. A relation may have more than one candidate key, and the different candidate keys may have a different number of attributes. In other words, you should not interpret ‘minimal’ to mean the super key with the fewest attributes.
A candidate key has two properties:
(i) in each tuple of R, the values of K uniquely identify that tuple (uniqueness)
(ii) no proper subset of K has the uniqueness property (irreducibility).
Primary Key
The primary key of a relation is a candidate key especially selected to be the key for the relation. In other words, it is a choice, and there can be only one candidate key designated to be the primary key.
Foreign keys
The attribute(s) within one relation that matches a candidate key of another relation. A relation may have several foreign keys, associated with different target relations.
Foreign keys allow users to link information in one relation to information in another relation. Without FKs, a database would be a collection of unrelated tables.
Example
CAR (State, LicensePlateNo, VehicleID, Model, Year, Manufacturer)
This schema has two keys:
K1 = { State, LicensePlateNo}
K2 = { VehicleID }
Both K1 and K2 are superkeys.
K3 = { VehicleID, Manufacturer} is a superkey, but not a key (Why?).
If a relation has more than one keys, we can select any one (arbitrarily) to be the primary key. Primary Key attributes are underlined in the schema:
CAR (State, LicensePlateNo, VehicleID, Model, Year , Manufacturer)

About the author

Victor Ikani

Ahmadderar is a website developer and SEO specialist. In his spare time, you'll catch him either reading books or attempting to write tutorials.

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