DAO: How To Attach to and Create QueryDefs on ODBC Tables

• Enterprise Edition of Microsoft Visual Basic, 32-bit only, for Windows, version 4.0

SUMMARY

This article describes how to attach and create Querydefs on external ODBC tables. The method for opening external ODBC tables is to attach the tables to an .mdb file.

Jet does not support named QueryDefs on a non-attached ODBC database. A non- attached ODBC database is one that is opened directly with the OpenDatabase method of the WorkSpace object without the use of an .mdb file.

If it is not appropriate for the application to attach the ODBC tables, it is possible to create Querydefs with no name to accomplish the procedure.

For additional information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q149055
   TITLE     : Jet Doesn't Support QueryDefs on a Non-Attached ODBC Table

MORE INFORMATION

The following is information from "Guide To Data Access Objects," Chapter 7, Data Access Choices, that explains this procedure:

In many cases, attaching a table to access external data is faster than opening a table directly, especially if the data is located in an ODBC database. In Visual Basic version 4.0, SQL Passthrough is used to query attached ODBC databases directly. Consider attaching external tables rather than opening them directly. Using external data in an ODBC database requires opening the external tables directly so performance is significantly slower when using the data.

Sample Program

The following example describes how to attach to and create a Querydef on an ODBC table using a "DSN-less" ODBC connection. With this procedure, it is not necessary to set up a DSN with the ODBC Admin utility.

1. Start a new project in Visual Basic. Form1 is created by default.

2. Add three Command buttons to Form1: Command1, Command2, Command3 by default.

3. Paste the following code in the General Declarations section of Form1:

         Dim db As Database
         Dim cn As String
         Private Sub Form_Load()
           cn = "odbc;driver={SQL Server};server=myserver;" & _
           "database=pubs;uid=myuid;pwd=mypwd"
           If Dir("mydb.mdb") <> "" Then
             ' database exists, so just open it.
             Set db = OpenDatabase(Name:="mydb", Exclusive:=False, _
               ReadOnly:=False, Connect:="")
           Else
             'database does not exist, create it and attach authors table.
             Set db = CreateDatabase(Name:="mydb", Connect:=dbLangGeneral, _
               Option:=dbVersion30)
             Dim td As TableDef
             Set td = db.CreateTableDef()
             td.Name = "Authors"
             td.SourceTableName = "Authors"
             td.Connect = cn
           End If
         End Sub
         Private Sub Command1_Click()
           Dim qd As QueryDef
           On Error Resume Next
           Set qd = db.QueryDefs("abc")  ' test for existence of querydef.
           If Error > 0 Then
             Set qd = db.CreateQueryDef(Name:="abc")
             qd.Connect = cn
             qd.SQL = "Select @@Version" 'native SQL Server
           End If
           Set qd = db.QueryDefs("xyz")  ' test for existence of querydef.
           If Error > 0 Then
             Set qd = db.CreateQueryDef(Name:="xyz")
             qd.Connect = cn
             qd.SQL = "Select * from titles" ' generic SQL.
           End If
           On Error GoTo 0
         End Sub
         Private Sub Command2_Click()
           Dim rs As Recordset
           Dim qd As QueryDef
           Set qd = db.QueryDefs("abc")
           Set rs = qd.OpenRecordset()
           Call displayResults(rs)
         End Sub
         Private Sub Command3_Click()
           Dim rs As Recordset
           Dim qd As QueryDef
           Set qd = db.QueryDefs("xyz")
           Set rs = qd.OpenRecordset()
           Call displayResults(rs)
         End Sub
         Sub displayResults(rs As Recordset)
           Dim f As Field, s As String, i As Integer
           For Each f In rs.Fields
             s = s & f.Name
           Next f
           Debug.Print s              ' print column headers.
           While Not rs.EOF And i < 5
             s = ""
             For Each f In rs.Fields
               s = s & f.Value
             Next f
             Debug.Print s            ' print first 5 rows.
             rs.MoveNext
             i = i + 1
           Wend
         End Sub
   
   

1. Press the F5 key to start the program.

2. Click the Command1 button to create the Querydefs. Click the Command2 and Command3 buttons to execute the Querydefs. Note that the first five rows of data appear in the Debug window.

REFERENCES

"Jet Database Engine Programmer's Guide", page 323, published by Microsoft Press.