Relational Database Management Systems

Table of Contents

1. What is a database management system?
2. What is a relational database management system?
3. Variations of SQL for RDBMSs
4. Constraints for RDBMSs

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What is a database management system?

• Database – logically modelled clusters of information/data
• Database management system (DBMS) – a computer program that interacts with a database
• Every DBMS has an underlying model that structures how data are stored and accessed.
  • e.g. relational database management system (RDBMS) ← relational data model

What is a relational database management system?

• Relational – data are organised into tables.
• Tables = relations
• A relation is a set of tuples.
• A tuple is a row in a table.
• Each tuple shares a set of attributes.
• An attribute is a column in a table.

Variations of SQL for RDBMSs

• SQL is used to manage and query data from relational databases.
• Many RDBMSs use their own particular dialect of SQL.
• The standard SQL exists.
  • SQL standards are jointly maintained by the American National Standards Institute (ANSI), the International Organisation for Standardisation (ISO), and the International Electrotechnical Commission (IEC).

Constraints for RDBMSs

Database administrator impose constraints on a table to limit what values can be entered into it. A constraint typically applies to one particular column. Some constraints can apply to an entire table. Commonly used constraints include:

• UNIQUE: to ensure that no two entries in a column are identical
• NOT NULL: to ensure that a column doesn’t have any NULL entries
• PRIMARY KEY: a combination of UNIQUE and NOT NULL
• FOREIGN KEY: A FOREIGN KEY is a column in one table that refers to the PRIMARY KEY of another table.
  • to link two tables together
  • Entries to the FOREIGN KEY column must already exist in the parent PRIMARY KEY column for the write process to succeed.
• CHECK: to limit the range of values that can be entered into a column