DEFINITION
Many definitions of databases exist, out of which we summarize the one stated by Lelutiu [0] : a database is a stored data collection that has two main characteristics : (1) it provides data independence by means of a data scheme and its corresponding data definition language , and (2) it provides access to the stored data ( large volumes of it ) by means of its physical data access level and its corresponding data manipulation language.
We have to make the following distinction : between a database and a DBMS as it is found moreover in the literature, which is the database management system, i.e. a computer program or more programs that cooperate in order to properly manage, query and maintain a database.
Having set this difference, we move on to the levels of abstraction that can be identified in a database. His approach may have been refined, but the structure J. D. Ullman identified in [2] stands even today, after more than 20 years :