Transcript Chapter 2
Chapter 2 Database System Concepts And Architecture Three Schema Architecture Internal Schema • Describes physical storage and access paths Conceptual Schema • Describes the structure of the database for all users External Schema • Describes pieces of the database available to subsets of users Three Schema Architecture Conceptual Schema Internal Schema Stored DB Advantages 1. Logical data independence User can change conceptual schema without have to change external Rule: You may change the conceptual organization of the db. If you change the mappings that produce the external view, programs that access the external view will work as before. 2. Physical Data Independence • You can add or remove indices without affecting the conceptual vies Database Languages In keeping with the 3-schema architecture we should have four distinct languages: 1. SDL: Storage Definition Language 2. DDL: Data Definition Language 3. VDL: View Definition Language 4. DML: Data Manipulation Language But, in practice DDL subsumes SDL, DDL, VDL In fact, in SQL DML and DDL are collapsed into a single language Important Distinction Set-A-Time vs. Record-At-A-Time In procedural languages, a single file retrival (read), produces a single record (struct or variable) SQL is a record-at-a-time language. A retrieval will often produce more than a single record—like a Google search. This is a distinction that must be confronted when SQL is embedded within C++. Many Possible Interfaces We’ll experiment with: SQL in character mode as DBA interface Canned parametric interfaces—SQL embedded in a procedural language A web-based interface Important to point out that Oracle includes utilities that lets applications programmers build forms. We won’t use this. DBMS Component Modules (p. 35) Database Organization Several ways to organize a database. We’ll use the simplest for our SQL queries and parametric programs Term 1 Term 2 DBMS (and DB) on a central server Term 3 … Could also separate query processing from database.This and the central design are referred to as 2-Tier architectures Client 1 Client 2 ODBC layer DBMS and DB minus query processing modules (for example) Client 3 … 3-Tier Architecture Gui Web Interface Web Server JDBC (in our case) A Java standard DB and DBMS