ACC2: Query by Form (QBF) Using Dynamic QueryDef

ID: Q117544


The information in this article applies to:


SUMMARY

Moderate: Requires basic macro, coding, and interoperability skills.

This article demonstrates how to use a form to specify the criteria for a query that is dynamically built by an Access Basic function. This technique is called query by form (QBF).

This article assumes that you are familiar with Access Basic and with creating Microsoft Access applications using the programming tools provided with Microsoft Access. For more information on Access Basic, please refer to the "Building Applications" manual.


MORE INFORMATION

You can use the QBF method to specify the criteria for filtering an existing query, but this method becomes complex as you add more fields to the form. For example, one such complication arises when you do not enter a value in one of the form's criteria fields.

A more flexible QBF method is to use an Access Basic function to dynamically create the SQL statement for a query that is deleted and recreated each time the function runs. This method does not use an empty criteria field as part of the query's WHERE condition.

Creating the Sample Function

Concatenating (joining) SQL statements in Access Basic requires that you "type cast" fields with special characters to instruct the Microsoft Jet database engine what field data types you are using. The following type-casting characters are used in this article:

   Field Type       Type-Casting Character
   ------------------------------------------
   Text             Single quotation mark (')
   Date             Number sign (#)
   Numeric          None 

The following sample SQL statements demonstrate the use of type casting.

NOTE: In the following sample statements, an underscore (_) at the end of a line is used as a line-continuation character. Remove the underscore from the end of the line when re-creating these statements.

   Text: "Select * from [Orders] where [ship city] = '" & [Mycriteria] _
   & "' ;"
   Date: "Select * from [Orders] where [Order Date] = #"& [Mycriteria] _
   & "# ;"
   Numeric: "Select * from [Orders] where [Employee ID] = "& [Mycriteria] _
   & ";" 

To create a sample function that uses the QBF technique to display the records matching the specified criteria, follow these steps:
  1. Open the sample database NWIND.MDB and create a new blank form.


  2. Add six text boxes to the form. Set the text boxes' Name property as follows:
    
          Text Box 1:
             Name: Customer ID
          Text Box 2:
             Name: Ship City
          Text Box 3:
             Name: Ship Country
          Text Box 4:
             Name: Employee ID
          Text Box 5:
             Name: Order Start Date
          Text Box 6:
             Name: Order End Date 


  3. Add a command button to the form and set the command button's Caption property as follows:
    Caption: Run Query


  4. Type the following code for the command button's OnClick event procedure.

    NOTE: In the following sample code, an underscore (_) at the end of a line is used as a line-continuation character. Remove the underscore from the end of the line when re-creating this code.
    
          Dim db As Database
          Dim QD As QueryDef
          Dim MyWhere As Variant
    
          Set db = dbengine.workspaces(0).databases(0)
    
          ' Delete the existing dynamic query; trap the error if the query does
          ' not exist.
          On Error Resume Next
          db.querydefs.Delete ("Dynamic_Query")
          On Error GoTo 0
    
          ' Note the single quotation marks surrounding the text fields _
          ' [Ship Country] and [Customer ID].
          ' Note that there are no type-casting characters surrounding the _
          ' numeric field [Employee Id].
          MyWhere = Null
          MyWhere = MyWhere & (" AND [Ship Country]= '" + _
             Me![Ship Country] + "'")
          MyWhere = MyWhere & (" AND [Customer Id]= '" + _
             Me![customer id] + "'")
          MyWhere = MyWhere & (" AND [Employee Id]= " + Me![Employee Id])
    
          ' The following section evaluates the Ship City criterion you enter.
          ' If the first or the last character of the criterion is the wildcard
          ' character (*), then the function uses the "LIKE" operator in the
          ' SQL statement instead of "=." Also note the single quotation
          ' marks surrounding the text field [Ship City].
    
          If Left(Me![Ship City],1) = "*" Or Right(Me![Ship City],1) = "*" Then
             MyWhere = MyWhere & (" AND [Ship City] like '" + Me![Ship City] _
             + "'")
          Else
             MyWhere = MyWhere & (" AND [Ship City] = '" + Me![Ship City]+"'")
          End If
    
          ' Note the number signs (#) surrounding the date field [Order Date].
          If Not IsNull(Me![order start date]) And _
                Not IsNull(Me![order end date]) Then
             MyWhere = MyWhere & (" AND [order date] between #" & _
                Me![order start date] + "# AND #" + Me![order end date] + "#")
          ElseIf IsNull(Me![order end date]) Then
             MyWhere = MyWhere & (" AND [order date] >= #" + _
                Me![order start date] + " #")
          ElseIf IsNull(Me![order start date]) Then
             MyWhere = MyWhere & (" AND [order date] <= #" + _
                Me![order end date] + " #")
          End If
    
          ' Remove the following MsgBox line if you do not want to display the
          ' SQL statement.
    
          MsgBox "Select * from orders " & (" where " + Mid(MyWhere, 6) + ";")
          Set QD = db.CreateQueryDef("Dynamic_Query", "Select * from _
             orders " & (" where " + Mid(MyWhere, 6) + ";"))
          DoCmd OpenQuery "Dynamic_Query" 


  5. View the form in Form view.


  6. To run the query, enter the following sample criteria in the text boxes, and then click the command button:
    
          Ship City text box: SE*
          Employee ID text box: 1
          Customer ID text box: CACTU
          Order Start Date text box: 1/1/94 


NOTE: The method in this example has the following features:


REFERENCES

For more information about the CreateQueryDef method, search for "CreateQueryDef," and then "CreateQueryDef Method (Data Access)" using the Microsoft Access Help menu.

For more information about SQL, search for "SQL: basics," and then "Retrieving Data Using SQL" using the Microsoft Access Help menu.

For more information about the QBF technique, search for "QBF," and then "Query by Form" using the Microsoft Access Help menu. Or, please see the following articles in the Microsoft Knowledge Base:

Q95931 ACC: How to Use the Query-by-Form (QBF) Technique

You can also obtain this article through Microsoft FastTips by ordering item number Q95931 from the FastTips Main Menu.


Keywords          : kbprg MdlQry 
Version           : 2.0
Platform          : WINDOWS 
Issue type        : kbhowto 

Last Reviewed: April 6, 1999