3. Let's Design a Database!

Seite 1

3. Lest's Design a Database!

What is the E-R model?

E-R (entity-relationship), try to define an entity in your data. Shows the relationship between entities The number of associations between entities is called cardinality.

How yo analyze the E-R model

Case 1: One-to-ne relationship

Case 2: One-to-many relationship

Case 3: Many- to-many relationship

Normalizing a table

Normalzation: process of tabulating data from the real world for a relational database.

Unnormalized form: is a table in which items appear more than once have not been removed.

First normal: refers to a simple two-dimensional table, resulting from division of the original, unnormalized table. The table is divides so that no items will appear more than once.

Second normal form refers to a table in wich a key that can identify data determines values in other columns. A value in called functionally dependent if that value determines values in other colums.

Third normal form: a table is divided so that a value is not determined by any non-primary key. A value is called transitively dependent if that value determines values in other columns indirectly .

Steps for designing a database

Conceptual schema: refers to a method that models the actual world. It's a way to determine the logical structure of a database.

Internal schema refers to a database viewed from the inside of a computer. It's a way to dtermine the physical structure of a database.

External schema refers to a database as viewed by users or applications.