Relational Database

A relational database implements the relational model.

A model is a way of abstracting complex real-world data into structures we can understand, talk about, manipulate and represent.

A relation is the definition of a table and the values stored in it as well as the rules on manipulating the rules.

Relational refers to tables and the relation tables have between each other.

Relational Databases - some rules

Everything is a relation:

All operations use the relational model
All data is represented and accessed as relations
Table and database structure is accessed and altered as relations

The system is unaffected by its implementation:

If the hardware changes
If the operating system changes
If the disks are replaced
If data is distributed

Relational model ≠ Entity-relationship model

An ER model helps model concepts, often as part of relational database design

Relational model ≠ SQL ≠ Most RDBMS implementations

Drawing a database

Entity

It's a thing we want to model and can be uniquely identified.

Attribute

Information that describes an aspect of an entity.

Relationship

A connection or dependency between two entities.

Entity-Relationship Diagrams

Entities are drawn in a box (quadrilateral).

Attributes are drawn inside an ellipse.

Relationships are drawn in a rhombus.

Speaking to databases

Basic SQL

SELECT
INSERT
UPDATE
DELETE

SELECT PlanetName FROM Planets SELECT keyword introduces/starts a query After the SELECT we specify the columns we want in the result (table). The FROM keyword tells us where to look.

SELECT PlanetName FROM Planets WHERE DayLength > 200;

Here the constraint for the field DayLength is 200. We'll see the PlanetName only for the rows which comply with the constraint.

CREATE TABLE Planets (
    PlanetName CHAR(8),
    DayLength INT,
    YearLength INT,
    PRIMARY KEY (PlanetName)
);

We use this to create a table with 3 field, one with character values up to 8 in length and two fields with integer values

In the relational model, every row has to be unique, this is done via the primary key in SQL.

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