Relational Database

SQL/Relational Database

Relational Databases 

Database management systems organize and structure data to retrieve and manipulate it by users and application programs. The data structures and access techniques a particular DBMS provides are called its data model. A data model determines both the "personality" of a DBMS and the applications for which it is particularly well suited.

SQL is a database language for relational databases that uses the relational data model. It eliminated the explicit parent or child structures from the database and represented all data in the database as simple row and column tables of data values. A relational database is a database where all data visible to the user is organized sharply as tables of data values, and where all database operations work on these tables.

File Management Systems

Before the introduction of database management systems, all data permanently stored on a computer system, such as payroll and accounting records, was stored in individual files. A file management system, usually provided by the computer manufacturer as part of the computer's operating system, kept the track of names and locations of the files. The file management system has no data model; it knew nothing about the internal contents of files. To the file management system, a file containing a word processing document and a file containing payroll data appeared the same.

The issues of maintaining huge file-based frameworks were driven within the late 1960s by the improvement of database management systems. The idea behind these systems was simple: take the definition of a file's content and structure out of the individual programs, and store it, together with the data, in a database. Using the information in the database, the DBMS that controlled it could take a much more active role in managing both the data and changes to the database structure.

Tables

The organizing principle in a relational database is the table, a rectangular, row/column arrangement of data values. The data or information for the database are stored in these tables. Tables are uniquely identified by their names and by their comprise of columns and rows. Columns contain the column name, data type, and any other attributes for the column. Rows contain the records or data for the columns. Here is a sample table called “Employee”

name, id, address, phone number, and email are the columns. The rows contain the data for this table:

A table can have any number of rows. A table of zero rows is perfectly legal and is called an empty table.