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 is used 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. An employee database is 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. This is useful when it is time to send out company Christmas card but is not useful for salary information.
user view: Specifies which users are permitted access to what data in a database.
logical design: The second stage in the DBLC: creating a logical schema, followed by data normalization.
physical design: The third stage in the DBLC: tweaking data design elements to optimize database performance.
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.
which will be discussed later on, is directly related to the logical schema.
In fact, the most significant end-product of logical design is the logical schema.
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).
which we will also discuss later on, is different from the physical schema. Physical design involves tweaking a database to optimize
performance, which does involve the designer.
The next lesson introduces a special class of tools often used in database design.
Before moving on to the next lesson, click the link to check your understanding of a databases three-tier architecture. Three Schema Architecture