The Database Design process

To manage databases you need database management systems (DBMS). A DBMS is an SQL application program that stores, retrieves, and modifies data in the database on request.

There are four main types of databases: hierarchical, network, relational, and more recently object relational. Very quickly the Relational database management systems (RDBMS) became very popular, especially for their ease of use and flexibility in structure. They are based on the following components: 1. Collections of relations or objects in which data is stored 2. Operators that produce relations acting upon other relations 3. Data integrity for accuracy and consistency Very quickly the Relational database management systems (RDBMS) became very popular, especially for their ease of use and flexibility in structure. They are based on the following components: Collections of relations or objects in which data is stored Operators that produce relations acting upon other relations Data integrity for accuracy and consistency. A relational database stores information in two-dimensional tables which represent relations. We showed in our previous example how we stored information about all the employees of a company. In our relational database, we created two tables to store different pieces of information about the employees such as their salary table, and their overtime month work table.. A relational database can contain one or many tables. A table is the basic storage structure of an RDBMS. A single row represents all data required for a particular entity or object. Each row in a table should be identified by a primary key, which allows no duplicate rows. The order of rows is not significant when data is stored. The row order is defined when the data is retrieved. A column or attribute in a table should contain the primary key.  The primary key identifies a unique value for each entity or object in the EMP table and generally can not be changed. A column or attribute that is not a key value represents one kind of data in a table. The order of columns is not significant during data is stored. The column order is defined when the data is retrieved. A foreign key is a column that defines how tables relate to each other. In the column of the foreign key of a table we keep the values of the primary key of another table. We cannot define foreign keys without existing primary (unique) keys  A data item or a table field can be found at the intersection of a row and a column with only one value in it. A data item or a table field may have no value in it. Thus, we can say that it has a null value.