1. A Table with a View: Database Queries
Chapter 15
A Table with a View:
Database Queries

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
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3. 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
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4. 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
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5. 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:
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7. 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)
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8. 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
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9. 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)
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10. 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
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11. 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
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12. Creating a Dean's View
Will include selected information from the physical tables
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13. 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
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14. 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
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15. Creating a Sport's Center View
Join Sports Center's view with its table and the administration table
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16. 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
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17. 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)
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18. SQL WHERE Clause
WHERE clause would set a condition (suppose the Dean only wanted to see students whose GPA is greater than 3.75)
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19. 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
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20. Entity Relationship Diagrams (cont'd)
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21. 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
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