FIX: Getting SELECT DISTINCT To Work Under ADO with SQL Server

• ActiveX Data Objects (ADO), versions 1.0, 1.5, 2.0, 2.1

SYMPTOMS

Under certain conditions with ActiveX Data Objects (ADO), SELECT DISTINCT in your SQL statement will not provide distinct records.

Any of the following situations produces incorrect results with SQL Server:

• You open a recordset with rs.open "select ....." with no command object.



• You open a recordset with the rs.open command.



• You open a recordset with Set rs= command.execute and the cursorlocation was set to rduseclient.


This happens either when you use the Recordset.Open method or when the recordset is obtained from a command object by setting the recordset equal to the return value of Command.Execute.

CAUSE

The reason this does not work with adUseClient is that ADO uses SQL Server's FOR BROWSE option on the SQL statement to get extended metadata in order to process updates. The use of this option causes SQL version 6.5 to ignore the DISTINCT keyword.

RESOLUTION

Here are the two possible techniques you can use to get SELECT DISTINCT to work as expected:

• Set the CursorLocation to adUseServer instead of adUseClient.



• When using the Open method with adUseClient you can work around this problem by using a command object and setting its ActiveConnection to the connection used in the Open statement.


The Steps to Reproduce Behavior section demonstrates the workarounds for both the Open and the Command methods.

STATUS

This problem is fixed with the release of Service Pack 5 for SQL Server 6.5 and SQL Server 7.x.

This problem still exists when using the recordset.open method when using SQL Server 6.5 with service packs less that sp5 and mdac 2.1 sp2.

The problem no longer exists when using the command.execute method in version 2.1 sp2 for MDAC even if the SQL Server service pack is less than sp5.

MORE INFORMATION

Steps to Reproduce Behavior

1. Create a standard .exe project, and from the Project menu, choose References, then select Microsoft ActiveX Data Objects 1.5 Library.



2. Paste the following code in the Declaration section of the form:




   Option Explicit

   Dim conn As New Connection
   'Dim com As New Command  'Uncomment to make DISTINCT work.
   Dim rs As New Recordset

 
   Private Sub Command1_Click()

     conn.open "mymachine", "sa", ""
     conn.cursorlocation = adUseClient  'Change to aduseServer.
     'com.ActiveConnection = conn  'Uncomment to make DISTINCT work.

     rs.open "select distinct state from authors", conn, _
         adOpenKeyset, adLockOptimistic
        MsgBox rs.RecordCount
   End Sub

   Private Sub Command2_Click()
     Dim counter As Integer
     counter = 0
        conn.Open "dsn=mymachine;uid=sa;pwd=;"
        com.ActiveConnection = conn
        conn.CursorLocation = adUseClient   'Comment this
        'conn.CursorLocation = adUseServer  'and uncomment this.
        com.CommandText = "select distinct state from authors"
        com.CommandType = adCmdText
        rs.CursorType = adOpenKeyset
        'cursortype will be adopenforwardonly with adUseServer.
        Set rs = com.Execute
        While Not rs.EOF
          Debug.Print rs!State
          counter = counter + 1
          rs.MoveNext
        Wend
        Debug.Print "--------------------"
        MsgBox counter  'recordcount return -1 with adUseServer
   End Sub 

3. Run the code and click Command1. Note the number of records returned. Change adUseClient to adUseServer in the CursorLocation. Run the code again and fewer records are returned if there are duplicate states.



4. Change aduseServer back to adUseClient. Uncomment the Dim of the command object and the code in Command1. Run the code again. You receive the correct number of records using this technique also.


NOTE: If you set the recordset properties ahead of time and use the syntax Rs.open com the recordcount returned will be incorrect. However, to get the correct recordcount using the Rs.Open method you must use a command object and set its active connection.


5. Run the code again and click Command2. Note the counter. Comment the CursorLocation for adUseClient and uncomment the adUseServer CursorLocation. Run the code and note the counter again.

REFERENCES

Additional query words:

Keywords : kbADO kbDatabase kbSQL kbGrpVBDB kbGrpMDAC kbDSupport kbADO210sp2fix kbMDAC210SP2
Version : WINDOWS:1.0,1.5,2.0,2.1
Platform : WINDOWS
Issue type : kbbug