### QUERYING A DATABASE

```QUERYING A DATABASE
By:
Dr.Ennis-Cole
OBJECTIVES:
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Learn how to use the Query window in
Design view
Create, run and Save queries
Define a relationship between two tables
Sort data in a query
Filter data in a query
Specify an exact match condition in a query
OBJECTIVES:
Change a datasheet’s appearance
 Use a comparison operator to match a
range of values
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Use the AND and OR logical operators
 Perform the calculations in a query
using calculated fields, aggregate
functions, and record group calculations
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QUERIES: An INTRODUCTION
data
 Access provides the following capabilities:
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Display selected fields and records from a table
Sort records
Perform calculations
Generate data for forms, reports and other
queries
– Update data in the tables of a database
– Find and display data from two or more tables
QUERY WINDOW
You use the Query window to create a
Query in Design view
 By using QBE, you give Access an
example of the information you are
requesting
 Access then retrieves the information
 Refer to Figures 3-1 and 3-2
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CREATING AND RUNNING A
QUERY
Drag CustomerNum from the Customer
field list
 Double click CustomerName
 Repeat the steps for the City,
OwnerName and First contact fields
 Click the Run button
 Save, type a name and close the
window
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DEFINING TABLE
RELATIONSHIPS
Define relationships between tables
 Use a common field to relate two tables
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This process is called a JOIN
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When you join tables that have a
common field, you can extract data from
them as if they were a single large table
ONE-TO-MANY
RELATIONSHIPS:
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Exists when one record in the first table
matches zero, one or many records in the
second table, or when one record in the
second table matches exactly one record in
the first table
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Access refers to the two tables that form a
relationship as the primary table and the
related table
ONE-to-MANY
RELATIONSHIPS:
Primary table: “One” table in a 1:M relation
 Related table:“Many” table in a1:M relation
 Because a related table is stored in two
tables, inconsistencies between the tables
can occur
 There are 3 major anomalies:
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– Deletion &
– Update anomalies
REFERENTIAL INTEGRITY
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Is a set of rules that Access enforces to
maintain consistency between related
tables
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Rules of RI:
– When you add a record to a related table, a
matching record must already exist in the
primary table
– If you attempt to change the value of the
primary key in the primary table, Access
prevents this change
– When you delete a record in the primary table,
Access prevents the deletion if matching
records exist in a related table
DEFINING A RELATIONSHIP
When two tables have a common field,
you can define a relationship between
them in the relationships window
 In this window, you can view or change
existing relationships, define and
rearrange the table layout
 Refer to Figures 3-6 & 3-7
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SORTING DATA IN A QUERY
Is the process of rearranging records in a
specified order
 Often you need to sort before displaying it
 When you sort, you do not change the
sequence of the records in the underlying
tables
 Only the records in the query are
rearranged
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SORTING DATA IN A QUERY
To sort, you must select the Sort key
 Sort keys can be text, number,
date/time, Currency, AutoNumber,
Yes/No
 They are not memo, OLE object or
 You sort keys in either ascending or
descending order
 Click the toolbar sort buttons to sort
quickly
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SORTING MULTIPLE FIELDS
IN DESIGN VIEW
Sort keys can be unique or non-unique
 When the sort key is non-unique,
records with the same sort key value
are grouped together
 But, they are not in a specific order
within the group
 To arrange these, you can specify a
Secondary Key
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SORTING A QUERY
DATASHEET
The first sort key field is called the
primary sort key
 A table has at most one primary key,
which must be unique
 Any field in a table can serve as a
primary sort key
 Access lets you select up to 10 different
sort keys
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SORTING & FILTERING DATA
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In the Query datasheet, select the field or
adjacent fields on which you want to sort
Click the Sort Ascending button or the Sort
Descending button on the Query Datasheet
toolbar
A Filter is a set of restrictions you place on the
records in an open datasheet or form to
temporarily isolate a subset of the records
In the datasheet, select all for filter
Click the Filter By Selection button the toolbar
DEFINING RECORD
SELECTION CRITERIA FOR
QUERIES
Just as you can display selected fields,
you can display selected records
 You must specify a condition as part of
the query
 A condition is a criteria that determines
which records are selected
 It consists of an operator and a value
 It could be a comparison operator or an
exact match
 Refer to Figures 3-17 through 3-20
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CHANGING A DATASHEET’S
APPEARANCE
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You can change characteristics of a datasheet
Click the Maximize button
Click the record selector to select the entire
sheet
Click Format, Font
Scroll, Click 8, Click OK button
Press and hold the shift key, then click the
mouse button, all columns are selected
Double-click the mouse button, all columns
are resized to their best fit
Click Save and Close
DEFINING MULTIPLE
SELECTION CRITERIA FOR
QUERIES
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Multiple selection requires you to use logical
operators
 They combine two or more conditions
 When you want to select a record, only if two
or more conditions are met, use the AND
Operator
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When you place conditions in different criteria
rows, you use the OR logical operator
 See Figure 3-25
The End !
```