QueryDef Object (DAO)

A QueryDef object is a stored definition of a query in a Microsoft Access database engine database.

Remarks

You can use the QueryDef object to define a query. For example, you can:

• Use the SQL property to set or return the query definition.

• Use the QueryDef object's Parameters collection to set or return query parameters.

• Use the Type property to return a value indicating whether the query selects records from an existing table, makes a new table, inserts records from one table into another table, deletes records, or updates records.

• Use the MaxRecords property to limit the number of records returned from a query.

• Use the ODBCTimeout property to indicate how long to wait before the query returns records. The ODBCTimeout property applies to any query that accesses ODBC data.

• Use the ReturnsRecords property to indicate that the query returns records. The ReturnsRecords property is only valid on SQL pass-through queries.

• Use the Connect property to make an SQL pass-through query to an ODC database.

You can also create temporary QueryDef objects. Unlike permanent QueryDef objects, temporary QueryDef objects are not saved to disk or appended to the QueryDefs collection. Temporary QueryDef objects are useful for queries that you must run repeatedly during run time but do not not need to save to disk, particularly if you create their SQL statements during run time.

You can think of a permanent QueryDef object in a Microsoft Access workspace as a compiled SQL statement. If you execute a query from a permanent QueryDef object, the query will run faster than if you run the equivalent SQL statement from the OpenRecordset method. This is because the Microsoft Access database engine doesn't need to compile the query before executing it.

The preferred way to use the native SQL dialect of an external database engine accessed through the Microsoft Access database engine is through QueryDef objects. For example, you can create a Microsoft SQL Server query and store it in a QueryDef object. When you need to use a non-Microsoft Access database engine SQL query, you must provide a Connect property string that points to the external data source. Queries with valid Connect properties bypass the Microsoft Access database engine and pass the query directly to the external database server for processing.

To create a new QueryDef object, use the CreateQueryDef method. In a Microsoft Access workspace, if you supply a string for the name argument or if you explicitly set the Name property of the new QueryDef object to a non–zero-length string, you will create a permanent QueryDef that will automatically be appended to the QueryDefs collection and saved to disk. Supplying a zero-length string as the name argument or explicitly setting the Name property to a zero-length string will result in a temporary QueryDef object.

To refer to a QueryDef object in a collection by its ordinal number or by its Name property setting, use any of the following syntax forms:

QueryDefs(0)

QueryDefs("name")

QueryDefs![ name]

You can refer to temporary QueryDef objects only by the object variables that you have assigned to them.

Link provided by: The UtterAccess community | About the Contributors

• Queries: Document SQL to Word

Example

This example creates a new QueryDef object and appends it to the QueryDefs collection of the Northwind Database object. It then enumerates the QueryDefs collection and the Properties collection of the new QueryDef.

Sub QueryDefX() 
 
   Dim dbsNorthwind As Database 
   Dim qdfNew As QueryDef 
   Dim qdfLoop As QueryDef 
   Dim prpLoop As Property 
 
   Set dbsNorthwind = OpenDatabase("Northwind.mdb") 
 
   ' Create new QueryDef object. Because it has a  
   ' name, it is automatically appended to the  
   ' QueryDefs collection. 
   Set qdfNew = dbsNorthwind.CreateQueryDef("NewQueryDef", _ 
         "SELECT * FROM Categories") 
 
   With dbsNorthwind 
      Debug.Print .QueryDefs.Count & _ 
         " QueryDefs in " & .Name 
 
      ' Enumerate QueryDefs collection. 
      For Each qdfLoop In .QueryDefs 
         Debug.Print "  " & qdfLoop.Name 
      Next qdfLoop 
 
      With qdfNew 
         Debug.Print "Properties of " & .Name 
 
         ' Enumerate Properties collection of new  
         ' QueryDef object. 
         For Each prpLoop In .Properties 
            On Error Resume Next 
            Debug.Print "  " & prpLoop.Name & " - " & _ 
               IIf(prpLoop = "", "[empty]", prpLoop) 
            On Error Goto 0 
         Next prpLoop 
      End With 
 
      ' Delete new QueryDef because this is a  
      ' demonstration. 
      .QueryDefs.Delete qdfNew.Name 
      .Close 
   End With 
 
End Sub 

This example uses the CreateQueryDef method to create and execute both a temporary and a permanent QueryDef. The GetrstTemp function is required for this procedure to run.

Sub CreateQueryDefX() 
 
   Dim dbsNorthwind As Database 
   Dim qdfTemp As QueryDef 
   Dim qdfNew As QueryDef 
 
   Set dbsNorthwind = OpenDatabase("Northwind.mdb") 
 
   With dbsNorthwind 
      ' Create temporary QueryDef. 
      Set qdfTemp = .CreateQueryDef("", _ 
         "SELECT * FROM Employees") 
      ' Open Recordset and print report. 
      GetrstTemp qdfTemp 
      ' Create permanent QueryDef. 
      Set qdfNew = .CreateQueryDef("NewQueryDef", _ 
         "SELECT * FROM Categories") 
      ' Open Recordset and print report. 
      GetrstTemp qdfNew 
      ' Delete new QueryDef because this is a demonstration. 
      .QueryDefs.Delete qdfNew.Name 
      .Close 
   End With 
 
End Sub 
 
Function GetrstTemp(qdfTemp As QueryDef) 
 
   Dim rstTemp As Recordset 
 
   With qdfTemp 
      Debug.Print .Name 
      Debug.Print "  " & .SQL 
      ' Open Recordset from QueryDef. 
      Set rstTemp = .OpenRecordset(dbOpenSnapshot) 
 
      With rstTemp 
         ' Populate Recordset and print number of records. 
         .MoveLast 
         Debug.Print "  Number of records = " & _ 
            .RecordCount 
         Debug.Print 
         .Close 
      End With 
 
   End With 
 
End Function 

The following example shows how to replace the Structured Query Language (SQL) statement in a saved query.

Sample code provided by: The Microsoft Access 2010 Programmer’s Reference | About the Contributors

‘To change the Where clause in a saved query  
Dim qdf as QueryDef
Dim db as Database
Set db = CurrentDB
Set qdf = db.QueryDefs(“YourQueryName”)
qdf.SQL = ReplaceWhereClause(qdf.SQL, strYourNewWhereClause)
set qdf = Nothing
set db = Nothing

Public Function ReplaceWhereClause(strSQL As Variant, strNewWHERE As Variant)
On Error GoTo Error_Handler

‘This subroutine accepts a valid SQL string and Where clause, and
‘returns the same SQL statement with the original Where clause (if any)
‘replaced by the passed in Where clause.
‘
‘INPUT:
‘ strSQL valid SQL string to change
‘OUTPUT:
‘ strNewWHERE New WHERE clause to insert into SQL statement
‘
    Dim strSELECT As String, strWhere As String
    Dim strOrderBy As String, strGROUPBY As String, strHAVING As String

    Call ParseSQL(strSQL, strSELECT, strWhere, strOrderBy, _
        strGROUPBY, strHAVING)

    ReplaceWhereClause = strSELECT &“”& strNewWHERE &“”_
        & strGROUPBY &“”& strHAVING &“”& strOrderBy

    Exit_Procedure:
        Exit Function

    Error_Handler:
        MsgBox (Err.Number & “: “ & Err.Description)
        Resume Exit_Procedure

End Function

About the Contributors

UtterAccess is the premier Microsoft Access wiki and help forum. Click here to join.

Wrox Press is driven by the Programmer to Programmer philosophy. Wrox books are written by programmers for programmers, and the Wrox brand means authoritative solutions to real-world programming problems.