This lesson explains how to project a typical relationships-entities model for a database to be used in our web projects. My approach is:
1. Define database entities (tables)
2. Identify attributes for tables
3. Define relationships and cardinality between the instances (records) of tables.
Step 1: define database entities
The first step when you project a database is to identify all entities (tables). For example if we want to project a simplified del.icio.us-like web site, our database will have these entities:
1. - USER (to store data about users, email, password, nickname,...)
2. - SITE (to store data about the sites added by the users)
These are only the main entities required from our project but, take a mind, that we will add other tables to store data about relationships between istances (records) of these tables in case of cardinality (M:M), many to many (see Step 3).
Step 2: define attributes
The next step is to define attributes for the tables USER and SITE. In this semplified example we will have something like this:
-----------
user_id_pk (Primary Key)
user_name
password
user_data (user signup date)
SITE
-----------
site_id_pk (Primary Key)
url
description
share_user (total number of users that share a site)
Step 3: define database relationships
Our simple application del.icio.us-like works in this way: an user add a site that can be shared by other users. The relationship's cardinality between USER table and SITE table is:
SITE > USER (M:M) - Many to Many (a site can be shared by many users).
In this case ( cardinality M:M) we have to add a new table (SHARE) that contains all possible combination between all instances of USER table and SITE table . In this new table, SHARE, to identify an user that share a site added by another user or by itself, we will add two Foreign Key:
-----------
share_id_pk (Primary Key)
user_id (Foreign Key > USER)
site_id (Foreign Key >SITE)
submitted_by (boolean: flag only if the current user has submitted the site)
Implement your database using SQL
Now, our database is ready to be implement with a DBMS (for example using MySQL). The next lesson will explains how to implement this database using SQL language.
Next Lessons:Create Tables and Relationships with SQL
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