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DB Implementation:
MS Access Queries
MS Access Queries
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Database Queries
Core DBA skill
From SQL to Query by Example (QBE)
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What does it do?
Find target information
• Retrieve, Filter, Sort, Aggregate/Summarize
Manipulate data
• Perform calculations
• Add, Change, Delete, Combine data in tables
• Assemble/Supply data for forms and reports
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How does it work?
Access translates QBE to SQL
SQL performs data manipulations based on Relational Algebra
Access queries create a dynaset (“live” view of table)
• changes made in data by query is reflected in underlying tables
Database Design
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Access Queries: Views
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Datasheet view
For displaying the result of the query
Useful for reviewing/validating the query
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Design View
For creating/modifying a query using drag & drop GUI (i.e., QBE)
Consists of Diagram Pane & Grid Pane
• Add tables/queries to the Diagram Pane
• Add fields to the Grid Pane (Field row)
Can sort/filter/compute by fields
• Sort row: set to Ascending/Descending
• Criteria row: use Expression to apply data filter
• Total row: compute (sum, min, max, count, etc.) of each field
Automatically generates SQL statements
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SQL View
For creating/modifying a query by manually writing SQL statements
Only way to create SQL-specific queries
• Union/Data-definition/Pass-through query
Database Design
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Access Queries: Basic Types
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Simple Query
Uses one table/query
To generate a subset (row/column) of a table
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Multi-table Query
Joins multiple tables/queries
To merge small chunks of data in normalized tables
• Linked tables are automatically linked in in the Query Design Grid
• Creating a link in the Query Design Grid does not permanently link tables
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Select Query
Selects records that meet given criteria
Does not change the data
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Parameter Query
Prompts for query criteria values (parameters) to run a dynamic query
Database Design
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Access Queries: Sort & Filter
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Datasheet View
Sorting
• Click column and right-click
Filtering
• Filter Tool (Home tab)
– Filter by Selection
– Filter by Form
– Advanced Filter/Sort
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Design View
Sort using the Sort Row
• Sort priority is from left to right for multiple sort
Filter using the Criteria Row
• Criteria in multiple rows make OR query
• Criteria in single row make AND query
• Criteria in a single cell
– AND/OR
– Wildcards (*) in Like and Between
» e.g. Like “A*”, Like “[A-C]*”, Like “*av*”, Like “ave?”, Not Like “A*”
» e.g. Between 1950 and 1960, >1960
Database Design
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Access Queries: Query Criteria
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Query Criteria Expressions
Expression
Returns
Between #1/1/99# and #12/31/99#
Dates from 1/1/99 to 12/31/99
In (“John”,”Mary”,”James”)
Records with John, Mary, or James
Is Null
Records with no entry
Like “Acc?ss”
‘Acc’ followed by any character, followed by ‘ss’
Like “*s”
Ends in ‘s’
Like “v*”
Starts with ‘v’
<1000
Less than 1000
1000
Equal to 1000
Like “[A-C]??”
Starts with A through C and has two more characters
????
Any four characters
Len([Surname])= Val(4)
Any Surname of 4 characters
Right([Surname],2)= “ss”
Any Surname ending in “ss”
Left([Surname],2)= “ac”
Any Surname starting with “ac”
Database Design
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Access Queries: Expressions
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Using expressions to create a calculated column
Enter expressions in blank column of query design view
• NAME: [Field1] operator [Field2]
Format the display in Format property of the field
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Examples
Simple math
• DiscountPrice: [Discount] * [StandardPrice]
String Concatenation
• Name: [FirstName] & “ ” & [Lastname]
Date & Time math
• DateDiff(Interval, BeginDate, EndDate)
– HireAge: DateDiff(“yyyy”,[BirthDate],[HireDate])
• DateAdd(Interval, Number, Date)
– RetireDate: DateAdd(“yyyy”,25,[HireDate])
Customized Sorting
• Switch(expr1, value1, expr2, value2, etc.)
– Switch([City]=“Seatle”, 1, [City]=“Redmond”, 2, etc.)
Database Design
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Access Queries: Dynamic Criteria
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Parameterized Queries
Dynamic query based on varying criteria value
•
e.g. retrieve books written by a given author
Enter the parameter name in square brackets in criteria
•
•
Do not use existing field names
Can use expressions
–
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e.g. Like "*" & [Last Name] & "*“
Query Criteria from Form Entries
Use a form to enter query parameter values
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good for multiple parameter entries
1.
2.
3.
Create a parameter query
Create a form with unbound input control for each of query parameters
Add a command button that will run the parameter query
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4.
Database Design
Run Query Action of Miscellaneous Category
Reference the form controls that hold parameter values from the query
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i.e. change the criteria to form controls
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Parameter name = fully qualified name of form control
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Access Queries: Joins
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Inner Join
Default join in Access (i.e. Natural Join)
Returns only the records where joined fields are equal in both tables
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Left Outer join
Returns all records from the left table
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Right Outer join
Returns all records from the right table
Database Design
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