Download Access Project 2

Querying a Database
Access Project 2
What is a Query?
 In general, a query is a form of questioning, in a line
of inquiry. A query may also refer to:
 A statement of information needs, typically keywords
combined with Boolean operators and other modifiers, in the
field of information retrieval.
 Simply put … a question represented in a way that the
database management system can understand (ie. Access)
 When you search for a book/article at the Library
or do an Internet search, you really are querying a
database.
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Why do we Need Queries?
 Natural languages (English) are too vague
 With complex questions, it can be hard to verify that the
question was interpreted correctly, and that the answer we
received is truly correct.
 Consider the question: Who are our best customers?
 We need a query system with more structure
 We need a standardized system so users and
developers can learn one method that works on any
(most) system.
 Query By Example (QBE)
 SQL
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Four Questions to Create a Query
 What output do you want to see?
Select the fields you wish to display.
 What do you already know?
Determine the constraints you already know and any additional
“hidden” constraints.
 What tables are involved?
Determine the tables involved in your query, even if the query
does not display data from those tables.
 How are the tables joined together?
Are the tables joined together automatically because of preset
keys or do you need to remove this auto-join and create a
different join?
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Query Organization
 Single table – we can query against one table
(ie. SELECT)
 Constraints – we can place constraints on the fields (ie.
=,>,<,<>, BETWEEN, LIKE, IS, AND, OR, NOT, NULL)
 Computations – we can perform a mathematical computation
(ie. +) or aggregation (ie. AVG)
 Groups/Subtotals – we can group records together
(ie. GROUP BY, WHERE, HAVING)
 Multiple Tables – we can join 2 or more tables together by key
fields (ie. INNER JOIN, OUTER JOIN, RIGHT JOIN, LEFT
JOIN)
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Access Project 2
 Dr. Gernaey, the Director of Continuing Education for Ashton James
College (AJC), is eager to query against the Client and Trainer
databases we setup in Project 1.
 Turn to page AC 67:
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What are the name, amount paid, and current due of client CP27
Which clients’ names begin with Fa?
Which clients are located in Lake Hammond?
Which clients have a current due of $0.00?
Which clients have an amount paid > $20,000?
Which clients of Trainer 48 have an amount paid > $20,000?
In what cities are all the clients located?
How many hours has each trainer worked so far this year?
What is the client number and name of each client and what is the number
and name of the trainer to whom each client is assigned?
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Using Text Data in Criterion (AC 75)
 You would use text data in criteria for a data field
whose data type is Text
 Note that sometimes fields that appear to be numeric
are actually text (ie. social security number)
 Simply type the text in the Criteria row below the
corresponding field name
 Access will automatically enclose text in quotation
marks. However, some DBMS require you to add the
quotes.
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Using a Wildcard in Criterion (AC 77)
Access provides 2 special wildcards:
 Wildcards are symbols that represent any character or
combination of characters
 Access automatically adds the LIKE operator and the quotation
marks to criteria that use wildcards.
 The Asterisk (*) represents any collection of characters
Fa*  The letters Fa, followed by any collection of characters
 The Question Mark (?) represents any single character
T?m  The letter T, followed by any single character followed by
the letter m, such as Tom or Tim
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Criteria for a Field Not in the Result (AC 78)
 Sometimes you may need to supply criteria for a field that
you do not want to appear in the results of a query
 Remove the check mark from the Show check box
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Using Numeric Data in Criterion (AC 82)
 You would use numeric data in criteria for a data field
whose data type is Numeric
 Enter the number in a criterion without dollar signs and
commas (ie. no formatting).
 An error message will display in Access if formatting is
entered with the number.
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Using Comparison Operators (AC 83)
 Unless specified, Access assumes that the criteria you enter
involves equality or an exact match (ie. =, LIKE)
 The order of the greater than or equal to (>=) and less than
or equal to (<=) operators is fixed; they cannot be reversed.
 Comparison operators are used with Numeric and Text data.
 The <> operator also can be used to indicate NOT
 What 5 comparison operators can be used in queries?
>
<
>=
<=
NOT
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Using Compound Criteria (AC 84)
 Sometimes you will have more than one criterion that the
data for which you are searching must satisfy
 There are two types of compound criteria:
AND criterion  each individual criterion must be true for the
compound criterion to be true
OR criterion  the compound criteria is true if either
individual criterion is true
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Using Compound Criteria (AC 84)
 When using the AND criterion:
The criteria must go on the SAME line in the Criteria area of
the grid.
 When using the OR criterion:
The criteria must go on SEPARATE lines in the Criteria area
of the grid.
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Sorting Data in a Query (AC 85)
 Sorting means ordering records in a particular way
 The sort key is the field or fields on which the records are
sorted
 You cannot sort on an asterisk
 To sort a query that includes an asterisk, add the individual sort
fields to the design grid and remove the check mark from the Show
check box
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Sorting on Multiple Keys (AC 88)
 If sorting on more than one field …
(such as Student Number and Name)
… the more important field is called the
major key (primary sort key)
 The secondary field (s) is called a
minor key (secondary sort key)
 Major keys must be to the left of minor key in design grid
(and in SQL)
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Sorting on Multiple Keys (AC 88)
 Example: City within State
 What is major sort key? What is minor sort key?
 Major sort key is State which follows the word “within”
 Minor sort key is City
 However, we wish to display the city before the state field.
 To display city before state but sort by city within state …
 Add the state field again to the design grid and remove the check
mark from the first state field (major sort key)
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Omitting Duplicates (AC 87)
 When you sort data, duplicates normally are included.
 For example, you could see “St. Louis” city appear more than once
in Maryville’s student database.
 We sometimes do not want duplicate values …
 There are 2 ways to omit duplicate values:
 Use a DISTINCT clause in SQL
 Change Unique Values property (to “Yes”) on the Property Sheet
 The Property Sheet is a window that contains various
properties of the object to which it pertains
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Creating a Top-Values Query (AC 89)
 Sometimes you may want to only show a specified number
of records or a percentage of records
 To do this run a Top-values query
 When you sort records …
 Limit results to records having the highest (descending sort) or
lowest (ascending sort) values
 Once the query has been run …
Close the query to reset the Top Values box to All
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Joining Tables (AC 92)
 A join is matching one table to another, based on some condition
Example:
Client  Trainer YIELDS Clients with assigned trainer
 In Access, a join line is automatically inserted when fields from the
2 tables have the same name
 If you do not want the auto-join, delete the join line
 Access does not add a join line when:
(1) the fields do not have matching names or
(2) the data type and data length are not identical on both fields
 If fields should be joined, drag the appropriate field in one of the
tables onto the matching field in the other table
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Types of Joins & Join Properties (AC 94)
 Inner Join (Access Default)
Only records that match are displayed in the result
 Right Outer Join
All the records in the “one” table in the relationship
will display
 Left Outer Join
All records in the “many” table in the relationship will
display
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Using Calculated Fields (AC 96)
 A calculated field is a field that can be computed from other
fields in a table (s)
 To enter a calculated field, enter the following 
- a name for the calculated field
- a colon
- the expression
 Field names in the expression must be enclosed in brackets [ ]
 Constants, such as .05, are not enclosed in brackets
 You may use the following mathematical operators: addition (+),
subtraction (-), multiplication (*), division (/), parentheses
 When more than one mathematical operator appears in an
expression, multiplication and division are performed before
addition and subtraction.
 To change the order of precedence, use parentheses.
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Changing a Format and Caption (AC 98)
 You can change the way items appear in query
results by changing the format
 Use the Property Sheet for the given field that you
wish to format
 You can change the desired property, such as
number of decimal places or the caption
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Calculating Statistics (AC 99)
 Access supports built-in statistics. These statistics
are called Aggregate functions.
An aggregate function is a function that performs some
mathematical function against a group of records.
 Include the function in the Total row on design grid
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Calculating Statistics (AC 99)
Aggregate Functions are listed below:
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COUNT
SUM
AVG (average)
MAX (largest value)
MIN (smallest value)
STDEV
(std. deviation)
 VAR (variance)
 FIRST
 LAST
Determines the number of items or values
Adds the values
Determines the arithmetic mean of values
Determines the greatest value
Determines the smallest value
Measures how widely values are dispersed
from the average value
Squares the standard deviation
Determines the first item or value
Determines the last item or value
 AVG, SUM, STDEV, VAR can be used only with numeric fields
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Calculating Statistics (AC 99)
 An aggregate function is different from a calculated
field
 A calculated field is a mathematical operation
performed on individual records
 An aggregate function is performed on groups of
records
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Use Criteria in Calculating Statistics (AC 101)
 Sometimes you only want records that satisfy a
certain criteria included in your statistics
 When you are using Aggregate Functions (ie. a Total
Row), you can use a Where Clause to restrict your
records
 For instance, we may want to see the Average
Amount Paid by clients where Trainer Number = 42
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Grouping (AC 103)
 Sometimes you may want to report your statistics in
combination with grouping
 Grouping means to create groups of records that
share common characteristics
 For instance, you may want to show the Average
Amount Paid for clients of each trainer
 Group By Trainer Number
 Avg Amount Paid
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Crosstab Queries (AC 104)
A Crosstab Query calculates a statistic (ie. sum, avg,) for
data that is grouped by two different types of information
Example:
Total Amount Paid  group on City and Trainer Number
One field appears down the side of datasheet (City)
Other field appears across the top (Trainer Number)
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Saving Queries
 When you save a query, you are saving the design of
the query, not the results.
 If you wish to save the results of a query, you can save the
results to a table.
 The query design you saved is run against the
current database.
 Note that if changes have been made to the data
since the last time you ran it, the results of the query
may differ.
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Querying a Database
Access Project 2
Any Questions?