Databases are characterized by a three-schema architecture because there are three different ways to look at them. Each schema is important to different groups in an organization. The graphic below illustrates this architecture and the groups most involved with each schema.

The notion of user views was mentioned earlier.
SQL, it was pointed out, is used both to create (DDL) and to invoke (DML) user views. Simply defined, user views specify which users are permitted access to what data in a database.
For example, an employee database might contain employee names, addresses, and phone numbers. It’s likely to contain employee salaries and other sensitive data as well. A user view can be created that enables a secretary, for example, to gain access to the names and addresses of employees--handy when it’s time to send out company Christmas cards--but not to salary information.

A database’s logical schema is its overall logical plan. This schema is developed with diagrams that define the content of database tables and describe how the tables are linked together for data access.
Database designers are responsible for creating the logical schema. Application developers and database administrators may find the logical schema useful for performing certain tasks.

The physical schema of a database refers to how data is stored on the computer on which it resides. Generally (but not always), physical storage is handled by the RDBMS. Neither users nor database designers get involved in the physical schema. Storage manipulation is a task for the systems programmer (if the RDBMS supports storage manipulation).

Before moving on to the next lesson, click the link to check your understanding of a database’s three-tier architecture.
Three Schema Architecture