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Chapter 2
The Relational Model 1:
Introduction, QBE, and
Relational Algebra
Concepts of Database Management, 4th Edition, Pratt & Adamski
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Objectives
 Describe
the relational model
 Understand
 Use
Query-by-Example (QBE)
Criteria in QBE
 Create
Calculated Columns in QBE
 Calculate
Statistics in QBE
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Objectives (con’t.)
 Sort
data in QBE
 Join
Tables in QBE
 Update
data using QBE
 Understand
relational algebra
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Relational Databases
 Collection



of tables
Each entity in own table
Attributes are fields (columns) in table
Relationships are common columns in two or
more tables
 Order
of rows and columns is immaterial
 Repeating
groups are not permitted
 Entries
with repeating groups are
unnormalized
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Relations
 Two






dimensional table in which:
Entries are single-valued
Each column (field or attribute) has a distinct
name
All values in a column represent the same
attribute
Order of columns is immaterial
Each row (record or tuple) is distinct
Order of rows is immaterial
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Query-by-Example (QBE)
 Query

Questions represented in a way the DBMS can
recognize and process
 QBE


Visual approach to writing queries
Used in MS-Access
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Simple Queries
Figure 2.3
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Simple Queries (con’t.)
Figure 2.4
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Query that Includes All Fields
Figures 2.5 – 2.6
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Query with Simple Criteria
Figures 2.7 – 2.8
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Query Using AND Criteria
Figures 2.9 – 2.10
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Query Using OR Criteria
Figures 2.11 – 2.12
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Query Using Two
Conditions on a Single Field
Figures 2.13 – 2.14
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Query Using Computed Field
Figures 2.15 – 2.16
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Query to Count Records
Figures 2.17 – 2.18
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Query to Calculate an Average
Figures 2.19 – 2.20
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Query to Sort Records
Figures 2.23 – 2.24
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Query to Sort on Multiple Keys
Figure 2.27
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Query to Sort on Multiple Keys (con’t.)
Figure 2.28
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Query to Join Tables
Figure 2.29
Concepts of Database Management, 4th Edition, Pratt & Adamski
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Query to Join Tables (con’t.)
Figure 2.30
Concepts of Database Management, 4th Edition, Pratt & Adamski
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Query to Restrict Records in a Join
Figures 2.31 – 2.32
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Update Query
Figure 2.35
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Delete Query
Figure 2.36
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Make-Table Query
Figure 2.37
Concepts of Database Management, 4th Edition, Pratt & Adamski
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Make-Table Query (con’t.)
Figure 2.39
Concepts of Database Management, 4th Edition, Pratt & Adamski
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Relational Algebra
 Theoretical
way of manipulating a relational
database to produce new tables
 Major commands

SELECT
 Retrieves

PROJECT
 Include

certain rows
certain columns
JOIN
 Pull
data from more than one table
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Customer & Sales Rep Tables
Figure 2.40
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Join of Customer and Sales Rep
Figure 2.41
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Outer Join of Customer and Sales Rep
Figure 2.42
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Normal Set Operations
 Union

of two tables
Result contains all rows that are in either the first
table, the second table, or both
 Intersection

Result contains all rows common to both
 Difference

of two tables
of tables
Result is the set of rows in one table but not the
other
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Normal Set Operations (con’t.)
 Product


Result contains Cartesian product
Obtained by concatenating every row in first table
with every row in second table
 Division

of two tables
Process
Result contains quotient
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Product of Two Tables
Figure 2.43
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Dividing One Table by Another
Figure 2.44
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