Chapter 15 A Table with a View: Database Queries
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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:
Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 15-6
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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)
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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)
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved Creating a Dean's View Will include selected information from the physical tables
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved Creating a Sport's Center View Join Sports Center's view with its table and the administration table
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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: SELECTList of fields FROMTable(s) WHEREConstraints on the rows
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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)
Copyright © 2006 Pearson Addison-Wesley. All rights reserved SQL WHERE Clause WHERE clause would set a condition (suppose the Dean only wanted to see students whose GPA is greater than 3.75)
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved Entity Relationship Diagrams (cont'd)
Copyright © 2006 Pearson Addison-Wesley. All rights reserved 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
Copyright © 2006 Pearson Addison-Wesley. All rights reserved
Copyright © 2006 Pearson Addison-Wesley. All rights reserved
Copyright © 2006 Pearson Addison-Wesley. All rights reserved