When porting a larger database application (130k LOC) that worked fine with .linksarray[0]. and InterBase to MS-SQL (6.5), I frequently got the error message 'connection is in use by another statement'.

At first, creating a new TDatabase for each TTable/ TQuery seemed to be necessary.

Then I found what was 'wrong' (not really wrong.. :-)

To speed up some of my queries, I had set the property Unidirectional to true. Delphi creates for such queries only one cursor (versus two for bidirectional queries or TTables). After removing the assignments of Unidirectional := true the error message disappeared and everything worked fine.

The following code resulted in the exception 'connection is in use by another statement':

// dataBaseNameS : string  is the name of the alias (MS-SQL 6.5)
begin
  Query1 := TQuery.Create (Application);
  With Query1 do
  begin
    DatabaseName := dataBaseNameS;
    SQL.Text := 'SELECT * FROM ABLESTOP';
    // the exception disappears if the following is removed
    Unidirectional := True;
    Open;
  end;
  ShowMessage ('ok')

  Table1 := TTable.Create (Self);
  With Table1 do
  begin
    DatabaseName := dataBaseNameS;
    TableName := 'COMPONENT_PLAN';
    UpdateMode := upWhereKeyOnly;
    Open
  end;

  Table1.Insert;
  Table1.FieldByName ('PARTNO').AsString := IntToStr (GetTickCount);
  Table1.FieldByName ('ID').AsString := 'WWxx';
  Table1.FieldByName ('VERSION').AsInteger := 1;
  // the exception will occurr in the next statement:
  //     "Connection is in use by another statement"
  Table1.Post;
end;