1.2. What Is a Database Management System?

A database management system (DBMS) is a computer program that allows creating new databases and interacting with existing databases with the help of corresponding user interfaces or with the help of statements expressed in a DB programming language. For creating new databases with data definition statements, a DB programming language contains a data definition language (DDL). For adding/updating/deleting data items with data manipulation commands, it contains a data manipulation language (DML). And for retrieving specific data items with DB queries, it contains a query language (QL).

An SQL DBMS allows managing relational databases, such as the movie database shown in Abbildung 1-1 above, with the help of SQL statements.

Abbildung 1-2. The user interface of the SQL DBMS "Microsoft Access".

Two examples of SQL DBMSs for desktop computers, shown in Abbildung 1-2 and Abbildung 1-3, are MS Access and the DB Browser for SQLite, which is developed by an Open Source software project. Both allow creating and altering DB tables, as well as inserting, updating and deleting records (table rows) without using SQL data definition and manipulation statements by providing special user interfaces. MS Access also allows entering and executing DB queries without using SQL.

Abbildung 1-3. The user interface of the SQL DBMS "DB Browser for SQLite".

A database system is a combination of a specific DB and a DBMS that makes this DB available to users who can interact with it by performing one of the following four data management operations:

  1. Retrieve one or more data items from the DB (and show them on the screen). For instance, retrieve the phone number of Tom Miller or show a list of all movies directed by Quentin Tarantino.
  2. Add one or more new data items to the DB. For instance, add two more movies directed by Tarantino.
  3. Update an existing data item in the DB. For instance, change the phone number of Tom Miller.
  4. Delete an existing data item from the DB. For instance, delete the phone directory entry of Jane Anselm.

The first operation, retrieving specific data items from a DB, can be performed either by browsing the DB contents or by issuing a DB query that asks for the wanted data items. The other operations, adding/updating/deleting data items, can be performed by with the help of corresponding data manipulation statements.