The document defines key concepts related to database schemas including: - A schema defines the structure and data in a database including entities, relationships, and constraints. - A schema diagram provides a visual representation of the database schema. - Database schemas have three levels: external, conceptual, and internal. The conceptual schema hides physical storage details.

1.  Schema:
- Logical structure of the database
 Database Schema
- Described in a formal language by the DBMS
- Be able to create a blueprint of how a database
will be constructed
 Schema Diagram
- A diagrammatic display of (some aspects of) a
database schema

2.  Schema Diagram Example:
* STUDENT, COURSE, e.g. are components of the schema.

3.  Definition:
* Set of specifications, rules, and processes that dictate how
data is stored in a database and how data is accessed by
components of a system.
* Defines the nature of the data, the structure of the data,
or how the data flows.
 3 Schema Architecture:
* External schema :
- Subset of the database
* Conceptual schema :
- Hides the details of physical storage structures :
entities, data types, relationships, operations, constraints.
* Internal schema :
- physical storage structure of the DB

4.  Definition:
* Techniques that allow data to be changed without affecting the
applications that process it.
 Logical data independence :
- Capacity to change the conceptual schema without having
to change external schema or application programs.
 Physical data independence :
- Change the internal schema without having to change the
conceptual (or external) schema.
- Internal schema may change to improve the performance
(e.g., creating additional access structure)

5.  Data Definition Language (DDL) :
* Used by the DBA and database designers to specify the conceptu
al schema of a database. In many DBMSs, the DDL is also used to
define internal and external schemas.
 Data Manipulation Language (DML) :
* Used to specify database retrievals and updates.
* COBOL, C or an assembly language.
 High level or non-procedural languages :
* E.g., SQL, are set-oriented and specify what data to retrieve than
how to retrieve. Also called declarative languages.
 Low level or procedural languages :
* Record-at-a-time; they specify how to retrieve data and
include constructs such as looping.

6.  Definition:
* Aggregate of means by which people interact with the database.
* Input allowing the users to manipulate a system, output allowing
the system to produce the effects of the users' manipulation.
 Example:
* Stand-alone query language interfaces.
* Programmer interfaces for embedding DML :
- Pre-compiler approach
- Procedure (subroutine) call approach
* User-friendly interfaces :
- Menu-based, Forms-based, Graphics-based,
Natural language etc.