Use an Analogy to understand Concept of Relational Databases
Relational databases let you use information in the same manner that you think about information.
For example, suppose you have an appointment with your attorney and you need to take a file to the office when you go to see the attorney.
You probably will make a notation in your calendar that you have the appointment and that you need to take the file with you when you go.
What you do not do is make a copy of the file, place it in your calendar, and then take that copy with you. What you are really doing is relating the appointment of the attorney with the file you need.
This is how relational databases work. When you retrieve the information from the database, you simply say, "I want this piece of information and those items that relate to it."
For example, with the attorney appointment, your request would be: " Tell me what time the appointment is to meet with Ms. Smith, and give me the file(s) I will need for the matters we will be discussing."
You just requested the related materials. That's the whole point behind SQL and relational databases. You ask for what you need and let the database do the work and go get it for you.
Relational Data Model
The relational data model was first introduced by Ted Codd of IBM Research in 1970 in a classic paper (Codd 1970), and it attracted immediate attention due to its simplicity and mathematical foundation. The model uses the concept of a mathematical relation, which looks somewhat like a table of values, as its basic building block, and has its theoretical basis in set theory and first-order predicate logic. In this module we discuss the basic characteristics of the model and its constraints.
The first commercial implementations of the relational model became available in the early 1980s, such as the SQL/DS system on the MVS operating system by IBM and the Oracle DBMS. Since then, the model has been implemented in a large number of commercial systems. Current popular relational DBMSs (RDBMSs) include DB2 and Informix Dynamic Server (from IBM), Oracle and Rdb (from Oracle), Sybase DBMS (from Sybase) and SQLServer and Access (from Microsoft).
In addition, several open source systems, such as MySQL and PostgreSQL make use of the relational model.