concepts of database management, fifth edition chapter 2: the relational model 1: introduction, qbe,...

Concepts of Database Concepts of Database Management, Fifth Management, Fifth

EditionEdition

Chapter 2: Chapter 2: The Relational Model 1:The Relational Model 1:

Introduction, QBE, andIntroduction, QBE, and

Relational AlgebraRelational Algebra

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Concepts of Database Management, 5th Edition

Objectives

Describe the relational model

Understand Query-by-Example (QBE)

Use 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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Premiere Products Sample Data

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Premiere Products Sample Data

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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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An Unnormalized Relation

Relational Database – a collection of relations

Unnormalized relation

When a structure satisfies all the properties of a relation except for the first item

Some of the entries contain repeating groups and thus are not single-valued

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Database Structure Representation

Write the name of the table followed by a list of all columns within parentheses

Each table should appear on its own line

Use the notation, tablename.columnname, with duplicate column names within a database

Using this combination qualifies column names

Primary key – the column or collection of columns that uniquely identifies a given row in a table

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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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Figure 2.3: An Example of Simple Queries

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Figure 2.4: Simple Queries (con’t.)

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Simple Criteria

Criteria – conditions that data must satisfy

Criterion – a single condition

To display specific query results, enter the condition in the appropriate column in the design grid

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Figures 2.7-2.8:Query with Simple Criteria

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Comparison (Relational) Operators Finds something other than an exact match

Comparison operators are:

= (equal to)

> (greater than)

< (less than)

>= (greater than or equal to)

<= (less than or equal to)

NOT (not equal to)

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Compound Criteria

Combines comparison operators

Many languages use AND or OR between the separate criteria

In an AND criterion, both criteria must be true

In an OR criterion, the overall criterion is true if either of the individual criteria is true

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Figures 2.9-2-10: Query Using AND Criteria

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Figures 2.11 – 2.12: Query Using OR Criteria

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Computed Fields

You can include calculated fields that are not in the database in queries

Computed field (calculated field) – a field that is the result of a calculation using one or more existing fields

In a query that uses computed fields, if a field name contains spaces you must enclose it in square brackets

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Figures 2.15-2.16: Query Using Computed Field

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Calculating Statistics

Count

Sum

Avg (average)

Max (largest value)

Min (smallest value)

StDev (standard deviation)

Var (variance)

First

Last

Built-in statistics (called aggregate functions in Access) include:

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Figures 2.19-2.20:Query to Calculate an Average

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Grouping

Grouping – creating groups of records that share some common characteristic

Functions can be used in combination with grouping where statistics are calculated for groups of records

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Sorting Sorting – listing records in a query’s results in a

particular way It is possible to sort using more than one field Sort Key – the field on which records are sorted Major sort key (primary sort key) – the more

important field Minor sort key (secondary sort key) – the less

important field Major sort key is on the left of the grid and the

minor sort key is on the right

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Figures 2.23-2.24:Query to Sort Records

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Figure 2.27: Query to Sort on Multiple Keys

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Figure 2.28:Query to Sort on Multiple Keys (con’t.)

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Joining Tables Queries to select data from more than one table

Join the tables based on matching fields in corresponding columns

In an Access query, a join line between matching fields in the two tables will be created indicating how the tables are related

When joining multiple tables

Add all the tables involved to the upper pane

Add the query results grid in the desired order

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Figure 2.29: Query to Join Table

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Figure 2.30: Query to Join Tables (con’t.)

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Update Query

Update query – a query that changes data

Makes a specified change to all records satisfying the criteria in the query

In Access, a new row is created that is used to indicate how to update the data selected by the query

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Figure 2.35: Update Query

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Delete Query

Queries can be used to delete one or more records at a time

Delete query – deletes all the records satisfying the criteria entered into the query

When you change the Query type to Delete Query, an extra row, called a Delete row, is added to the design grid

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Figure 2.36: Delete Query

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Make-Table Query

Queries can be used to create a new table in either the current database or in a separate database

Make-table query – creates a new table using the query results

The data added to the new table is separate from the original table in which it appears

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Figure 2.37: Make-Table Query

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Figure 2.39: Make-Table Query (con’t.)

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Relational Algebra

Theoretical way of manipulating a relational database to produce new tables

Relational algebra includes operations that act on existing tables to produce new tables

Retrieving data using relational algebra involves issuing relational algebra commands to operate on existing tables to form a new table containing the desired information

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Major commands SELECT – retrieves certain rows from existing

table

PROJECT – causes only certain columns to be included in the new table

JOIN

Allows extraction of data from more than one table

Rows in new table will be the concatenation (combination) of a row from the first table and a row from the second

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The Join Command Continued If there is a row in one table that does not match

any row in the other table, that row will not appear in the result of the join

PROJECT command can be used to restrict the output from the join

Natural join – joins the records from each original table that is common to both tables

Outer join – joins the records from each original table including the records not common to both tables

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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

Union compatible – tables are union compatible if they have the same number of columns and their corresponding columns represent the same type of data

Intersection of two tables

Result contains all rows common to both

Use the INTERSECT command

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Normal Set Operations (con’t.) Difference of tables

Result is the set of rows in one table but not the other

Performed by using the SUBTRACT command

Product of two tables

Mathematically called the Cartesian product

Obtained by concatenating every row in first table with every row in second table

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Normal Set Operations (con’t.)

Division Process

Best illustrated by considering the division of a table with two columns by a table with a single column

Result contains quotient

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Summary Relation: two-dimensional table in which the

entries are single-valued Relational database: collection of relations Field name: qualified by preceding it with the table

name and a period Table’s primary key: field or fields that uniquely

identify a given row within the table Query-By-Example (QBE): visual tool for

manipulating relational databases Created by completing on-screen forms

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Summary (con’t.) Queries can be used to select specific records based

on certain criteria

A make-table query creates a new table using the query results

Relational Algebra - a theoretical way of manipulating a relational database to produce new tables

The normal set operations include the union, intersection, difference and product of two tables and the division process

concepts of database management, fifth edition chapter 2: the relational model 1: introduction, qbe,...

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