1. Concepts of Database Management, Fifth Edition
Chapter 2:
The Relational Model 1:
Introduction, QBE, and
Relational Algebra

2. Objectives Describe the relational model
Understand Query-by-Example (QBE)
Use Criteria in QBE
Create Calculated Columns in QBE
Calculate Statistics in QBE
Concepts of Database Management, 5th Edition

3. Objectives (con’t.) Sort data in QBE Join Tables in QBE
Update data using QBE
Understand relational algebra
Concepts of Database Management, 5th Edition

4. 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
Concepts of Database Management, 5th Edition

5. Premiere Products Sample Data
Concepts of Database Management, 5th Edition

6. Premiere Products Sample Data
Concepts of Database Management, 5th Edition

7. 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
Concepts of Database Management, 5th Edition

8. 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
Concepts of Database Management, 5th Edition

9. 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
Concepts of Database Management, 5th Edition

10. Query-by-Example (QBE)
Questions represented in a way the DBMS can recognize and process
QBE
Visual approach to writing queries
Used in MS-Access
Concepts of Database Management, 5th Edition

11. Figure 2.3: An Example of Simple Queries
Concepts of Database Management, 5th Edition

12. Figure 2.4: Simple Queries (con’t.)
Concepts of Database Management, 5th Edition

13. 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
Concepts of Database Management, 5th Edition

14. Query with Simple Criteria
Figures :
Query with Simple Criteria
Concepts of Database Management, 5th Edition

15. 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)
Concepts of Database Management, 5th Edition

16. 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
Concepts of Database Management, 5th Edition

17. Figures 2.9-2-10: Query Using AND Criteria
Concepts of Database Management, 5th Edition

18. Figures 2.11 – 2.12: Query Using OR Criteria
Concepts of Database Management, 5th Edition

19. 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
Concepts of Database Management, 5th Edition

20. Figures 2.15-2.16: Query Using Computed Field
Concepts of Database Management, 5th Edition

21. Calculating Statistics
Built-in statistics (called aggregate functions in Access) include:
Count
Sum
Avg (average)
Max (largest value)
Min (smallest value)
StDev (standard deviation)
Var (variance)
First
Last
Concepts of Database Management, 5th Edition

22. Query to Calculate an Average
Figures :
Query to Calculate an Average
Concepts of Database Management, 5th Edition

23. 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
Concepts of Database Management, 5th Edition

24. 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
Concepts of Database Management, 5th Edition

25. Figures 2.23-2.24: Query to Sort Records
Concepts of Database Management, 5th Edition

26. Figure 2.27: Query to Sort on Multiple Keys
Concepts of Database Management, 5th Edition

27. Query to Sort on Multiple Keys (con’t.)
Figure 2.28:
Query to Sort on Multiple Keys (con’t.)
Concepts of Database Management, 5th Edition

28. 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
Concepts of Database Management, 5th Edition

29. Figure 2.29: Query to Join Table
Concepts of Database Management, 5th Edition

30. Figure 2.30: Query to Join Tables (con’t.)
Concepts of Database Management, 5th Edition

31. 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
Concepts of Database Management, 5th Edition

32. Figure 2.35: Update Query
Concepts of Database Management, 5th Edition

33. 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
Concepts of Database Management, 5th Edition

34. Figure 2.36: Delete Query
Concepts of Database Management, 5th Edition

35. 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
Concepts of Database Management, 5th Edition

36. Figure 2.37: Make-Table Query
Concepts of Database Management, 5th Edition

37. Figure 2.39: Make-Table Query (con’t.)
Concepts of Database Management, 5th Edition

38. 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
Concepts of Database Management, 5th Edition

39. 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
Concepts of Database Management, 5th Edition

40. 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
Concepts of Database Management, 5th Edition

41. Normal Set Operations Union of two tables Intersection 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
Concepts of Database Management, 5th Edition

42. 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
Concepts of Database Management, 5th Edition

43. 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
Concepts of Database Management, 5th Edition

44. 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
Concepts of Database Management, 5th Edition

45. 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, 5th Edition