Download Query-by-Example (QBE) - gozips.uakron.edu

The University of Akron
Dept of Business Technology
Computer Information Systems
2440: 180
Database Concepts
Instructor: Enoch E. Damson
The Relational Model:
Query-By-Example (QBE)
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
The Relational Model: QBE
2
Figure 2.3: An Example of Simple Queries
The Relational Model: QBE
3
Figure 2.4: Simple Queries…
The Relational Model: QBE
4
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
The Relational Model: QBE
5
Query with
Simple Criteria
The Relational Model: QBE
6
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)
The Relational Model: QBE
7
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
The Relational Model: QBE
8
Query Using AND Criteria
The Relational Model: QBE
9
Query Using OR Criteria
The Relational Model: QBE
10
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
The Relational Model: QBE
11
Query Using Computed Field
The Relational Model: QBE
12
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
The Relational Model: QBE
13
Query to Calculate
an Average
The Relational Model: QBE
14
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
The Relational Model: QBE
15
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
The Relational Model: QBE
16
Query to Sort Records
The Relational Model: QBE
17
Query to Sort on Multiple Keys
The Relational Model: QBE
18
Query to Sort on Multiple Keys…
The Relational Model: QBE
19
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
The Relational Model: QBE
20
Query to Join Table
The Relational Model: QBE
21
Query to Join Tables…
The Relational Model: QBE
22
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
The Relational Model: QBE
23
Update Query
The Relational Model: QBE
24
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
The Relational Model: QBE
25
Delete Query
The Relational Model: QBE
26
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
The Relational Model: QBE
27
Make-Table Query
The Relational Model: QBE
28
Make-Table Query…
The Relational Model: QBE
29