Database Management System (DBMS)

Introduction

Almost every organization we deal with, a college, a bank, a hospital, a telecom company, survives on the information it keeps about students, accounts, patients or customers. In the early days this information was kept in ordinary files, one file per application, and every program had to know exactly how those files were laid out. That file based approach worked for small problems but broke down badly as data grew: the same fact was copied into many files, changing one copy left the others wrong, and two programs could not safely use the same file at once. A Database Management System was created to solve exactly these problems, and this chapter studies what it is, how it lets us describe and change data, how data is modelled and organized, and how it is kept secure.

Data, database and DBMS

Data are raw facts, plain figures, characters or symbols that on their own carry little meaning, for example the values 1, Rubina, 2081-04-05 or 5.4. When data are processed and arranged so that they help us take decisions, they become information.

A database is a collection of interrelated data about the objects or entities of an organization, stored together in an organized (usually tabular) form so that it can be shared and retrieved easily. The data are interrelated: a student's marks relate to that student's record, which relates to the class they belong to.

A Database Management System (DBMS) is the software that defines, stores, manipulates and manages a database. It sits between the user (or the application program) and the physical data files, and it lets us create the structure of the data, insert and update records, retrieve exactly the records we want, and control who may do what. Popular examples are Oracle, MySQL, Microsoft SQL Server, PostgreSQL and Microsoft Access.

Field, record, table and object

The relational way of organizing data, which is what NEB studies, uses a few basic terms:

• A field (or attribute) is a single item of data, one column of a table, such as Name or Roll_No.
• A record (or tuple) is one complete set of related fields, one row of the table, describing one entity such as a single student.
• A table (or relation) is a collection of records of the same kind arranged as rows and columns.
• An object is any real world thing (a student, a book, an account) that the database keeps data about; each object usually becomes a table.

Keys

A key is a field, or a group of fields, used to identify records uniquely and to link tables together.

• Primary key: an attribute (or set of attributes) whose value uniquely identifies each record in a table, and which can never be duplicated or left null. A student's registration number is a natural primary key.
• Candidate key: any attribute or combination of attributes that is capable of acting as a primary key, that is, of identifying rows uniquely. A table may have several candidate keys; the designer chooses one of them to be the primary key.
• Alternate key: a candidate key that was not chosen as the primary key.
• Foreign key: an attribute in one table whose value refers to the primary key of another table. Foreign keys are what create the relationship between two tables and they enforce referential integrity, the rule that you cannot refer to a record that does not exist.