ACC: Query by Form (QBF) Using Dynamic QueryDef (95/97)

MORE INFORMATION

You can use the QBF method to specify the criteria for filtering an existing query, but this method can become complex when 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 a Visual Basic function to dynamically create the SQL statement for a query that is deleted and recreated each time the function runs. This method avoids using an empty criteria field as part of the query's WHERE condition.

Creating the Sample Visual Basic Function

Concatenating (joining) SQL statements in Visual 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 [ShipCity] = '" & [Mycriteria] _
   & "' ;"
   Date: "Select * from [Orders] where [OrderDate] = #"& [Mycriteria] _
   & "# ;"
   Numeric: "Select * from [Orders] where [EmployeeID] = "& [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 Northwind.mdb and create a new, blank form based on the Orders table.
2. Add six text boxes to the form. Set the Name property of each text box 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 following properties:

         Name: cmdRunQuery
         Caption: Run Query
   					

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

         Private Sub cmdRunQuery_Click()
         Dim db As Database
         Dim QD As QueryDef
         Dim where As Variant
   
         Set db = CurrentDb()
   
         ' 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 [CustomerID].
         ' Note that there are no type-casting characters surrounding the
         ' numeric field [EmployeeID].
   
         where = Null
         where = where & " AND [ShipCountry]= '" + Me![Ship Country] + "'"
         where = where & " AND [EmployeeID]= " + Me![Employee Id]
   
         'NOTE: In Microsoft Access 97, when you use the plus sign (+) in an
         'expression in which you are concatenating a variable of the numeric
         'data type, you must use parenthesis around the syntax, as in the
         'following example:
         '
         '   where = where & (" AND [EmployeeID]= " + Me![Employee Id])
         '
         'You must also use a conversion function to make sure that the proper
         'conversion (to either NULL or String) takes place.
   
         ' The following section evaluates the ShipCity criteria you enter.
         ' If the first or last character of the criteria 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 [ShipCity].
         If Left(Me![Ship City],1) = "*" Or Right(Me![Ship City],1) = "*" Then
            where = where & " AND [ShipCity] like '" + Me![Ship City] + "'"
         Else
            where = where & " AND [ShipCity] = '" + Me![Ship City] + "'"
         End If
   
         ' Note the number signs (#) surrounding the date field [Order Date].
         If Not IsNull(Me![Order End Date]) Then
            where = where & " AND [OrderDate] between #" + _
            Me![Order Start Date] + "# AND #" & Me![Order End Date] & "#"
         Else
            where = where & " AND [OrderDate] >= #" + Me![Order Start Date] _
            + " #"
         End If
   
         ' Remove the following MsgBox line if you do not want to display the
         ' SQL statement.
         ' NOTE: The Mid function is used in the following MsgBox function to
         ' remove the word AND that follows the first Where clause. If you do
         ' not use the Mid function, the SQL statement contains the word AND
         ' at the beginning of the WHERE clause, for example:
         '
         '   Select * from Orders where AND [CustomerID] = 'CACTU'
   
         MsgBox "Select * from Orders " & (" where " + Mid(where, 6) & ";")
         Set QD = db.CreateQueryDef("Dynamic_Query", _
         "Select * from orders " & (" where " + Mid(where, 6) & ";"))
         DoCmd.OpenQuery "Dynamic_Query"
         End Sub
   					

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:

   Customer ID text box: CACTU
   Ship City text box: BU*
   Employee ID text box: 8
   Order Start Date text box: 1/1/94

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

• The AND operator is used to evaluate the text box criteria. For example, if you enter "BONAP" in the Customer ID text box and "1" in the Employee ID box, the resulting query displays records in which [CustomerID] = BONAP AND [EmployeeID] = 1.
• If you enter an asterisk (*) at the beginning or end of a value in the Ship City text box, the asterisk is interpreted as a wildcard and the LIKE operator is used in the SQL statement. For example, if you enter "Sea*" in the Ship City text box, the resulting query displays records for which the ShipCity is Seattle.
• If you enter a criteria in the Order Start Date text box, but not in the Order End Date box, the resulting query searches for everything after the specified start date instead of searching for a range of dates.
• After you enter the criteria on the form and click the command button, the query is built and run. You can also use this function to print a report based on the query.
• The SQL statement built for the dynamic query is displayed in a message box.
• If you do not enter any criteria, the resulting query returns all records in the table.