Databases

The central concept of a database is that of a collection of records, or pieces of knowledge. Typically, for a given database, there is a structural description of the type of facts held in that database: this description is known as a schema. The schema describes the objects that are represented in the database, and the relationships among them.

Primary Key:
This is an index that cannot be NULL, Primary Keys are used in building relationships between tables in a database. (an index is automatically created on the primary key). The difference between primary and ordinary keys is that there can be multiple keys, but only one primary key.

Unique Key:
Unique and Index are same, the difference is, in Unique, duplicate are not allowed in any circumstances and that is enforced by database server.

Database tutorial for Data Warehouse

Data Mining is the process of extracting knowledge hidden from large volumes of raw data.

Before Designing Database for Data warehouse you should know-

What is a Data Warehouse? What is a Data Mart?

What is a Star Schema Database?

Why do I want/need a Star Schema Database?

The Star Schema looks very denormalized. Won’t I get in trouble for that?

Reasons for Dirty Data

Dummy Values
Absence of Data
Multipurpose Fields
Cryptic Data
Contradicting Data
Inappropriate Use of Address Lines
Violation of Business Rules
Reused Primary Keys,
Non-Unique Identifiers
Data Integration Problems

Biggest databases

Company	Database size (GB)	DBMS
France Telecom	29232	Oracle
AT&T	26269	Daytona
SBC	24805	Teradata