Chapter 15 A Table with a View: Database Queries

Chapter 15

A Table with a View:Database Queries

Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 15-2

The Physical Database

• Redundancy is Bad, Very Very Very Bad– Never duplicate information in

database table

• To avoid inconsistency among copies– We might change the information in one place,

and forget to change it in another

• Inconsistent data is known as garbage– Worse than having no data at all


The Physical Database (cont'd)

• Keep Only One Copy of Information

– Avoiding duplication promotes internal consistency, but does not ensure that the information is correct

• Database information may be needed in several places

– Best to keep a master list and allow access


The Physical Database (cont'd)

• Keep a Separate Table and a Key

– Rather than repeating information, keep a separate table keyed with a unique identifier (foreign key)

– When information is needed, it can be looked up using the foreign key


The Database Schema

• The metadata of a database's tables is called its schema or scheme

– Structure and design

• Imagine a college having two tables defined in its schema, Student and Home_Base:



Connecting Database Tables by Relationship

• The two tables are separate, but not independent

• The common Student_ID field connects them

– There is a relationship between the two entities• Correspondence between rows

• Relationships are part of the metadata

• This is a two-way relationship (we can find the address for any student, or the student for any address)


Reconstruction Using Join

• The relationship between the two tables allows us to construct a single table (Master_List) containing the combined information from both tables

– Use the natural join operation from Chapter 14


Designing a Database Schema

• Suppose the Dean's office and the sports center also need address information

– Define tables without using address, but with Student_ID as primary key

– Each new table has one-to-one relationship with Home_Base table, and with Student table (so the Dean or sports center could look up the students' names)


Physical versus Logical Database

• From our set of four tables, we can create other tables customized to different campus units

– Logical database — does not physically exist• Created fresh every time it is needed, using current

values in the physical database

• Logical databases contain duplicate information, so we don't want to store them and create redundancy

• Personalized logical databases (database views) allow every user group to see the database in a different way


Logical Database: Creating Views

• Views are the logical tables constructed by database operations from the physical table

• The operations that create views are called database queries– Natural join is a query

• Every named table of a database is either a physical table, stored on the disk, or a logical table created by a query


Creating a Dean's View

• Will include selected information from the physical tables


Join Three Tables into One

• First step: Note that Dean's View contains information from three tables

• Join operation associates the information for each student

• You'd have all the information from all three tables for each student


Trim the Table

• Retrieve only the columns the Dean wants to see

• Join-then-trim strategy is a standard approach

– A super table is formed by joining several physical tables

– Then they are trimmed down to keep only the information that is of interest to the user


Creating a Sport's Center View

• Join Sports Center's view with its table and the administration table


A Query Language: SQL

• SQL (Structured Query Language):

– Widely used standard language

– Provides specific query structure for techniques like join-then-trim

– SQL varies (dialects) by vendor, but simple queries are roughly the same:

• SELECT List of fields• FROM Table(s)• WHERE Constraints on the rows


SQL ON Clause

• Following the SELECT is the list of fields

• the FROM has to Joins that have been grouped together (which field they are joined ON)


SQL WHERE Clause

• WHERE clause would set a condition (suppose the Dean only wanted to see students whose GPA is greater than 3.75)


Entity Relationship Diagrams

• Creating new tables involves relationships

• The point of identifying relationships in a database schema is to indicate how the information is connected, and joins make these connections

• If there are relationships, they are likely to be applied when building the logical database

• Database administrators diagram the relationships to make the structure clear


Entity Relationship Diagrams (cont'd)


Entity Relationship Diagrams (cont'd)

• One-to-One Relationships– Any row in the first entity is associated with at

most one row in the other entity

• Many-to-One Relationships– Many of the rows of the first entity can be

associated with a single row in the second entity

• These types of relationships can de shown in different ways in ER diagrams




Documents

Chapter 15 A Table with a View: Database Queries