How to sort columns in a TDBGrid

Problem/Question/Abstract:

How to sort columns in a TDBGrid

Answer:

Many professional applications will display data in grid fields and allow you to sort on any one of the columns simply by clicking on the column header. Although what is proposed here is not the best way to accomplish this, it is a fairly simple way to mimic the same behavior. The key hurdle in this problem is the DBGrid itself. It has no OnClick or OnMouseDown events, so it really was not designed to capture this kind of input. It does provide an OnDoubleClick, but this really doesn't work too well. What we need is a way to make the column headers clickable. Enter the THeaderControl component. THeaderControl is a component that comes in Delphi 2.0 and provides the basic functions that we want. It can detect clicks on its individual panels, and the panels even go up and down when pressed (like a button). The key is to connect the THeaderControl to the DBGrid. Here is how it is done:

First, start a new application. Drop a THeaderControl on the form. It will automatically align to the top edge of the form. Now drop a DBGrid on the form and set its Align property to alClient. Next, add a TTable, and TDataSource component. Set the Tables DatabaseName property to DBDEMOS and its TableName to EVENTS.DB. Set the DataSource's DataSet property to point at Table1 and the DBGrid's DataSource property to point to DataSource1. Set Table's Active property to False in case it has been turned on.

Now we need to setup the THeaderControl component to look like the DBGrid's column headers. his will be done in code in the Form's FormCreate method. DoubleClick on Form1's OnCreate event and enter the following code:

procedure TForm1.FormCreate(Sender: TObject);
var
  TheCap: string;
  TheWidth, a: Integer;
begin
  DBGrid1.Options := DBGrid1.Options - [dgTitles];
  HeaderControl1.Sections.Add;
  HeaderControl1.Sections.Items[0].Width := 12;
  Table1.Exclusive := True;
  Table1.Active := True;
  for a := 1 to DBGrid1.Columns.Count do
  begin
    with DBGrid1.Columns.Items[a - 1] do
    begin
      TheCap := Title.Caption;
      TheWidth := Width;
    end;
    with HeaderControl1.Sections do
    begin
      Add;
      Items[a].Text := TheCap;
      Items[a].Width := TheWidth + 1;
      Items[a].MinWidth := TheWidth + 1;
      Items[a].MaxWidth := TheWidth + 1;
    end;
    try
      Table1.AddIndex(TheCap, TheCap, []);
    except
      HeaderControl1.Sections.Items[a].AllowClick := False;
    end;
  end;
  Table1.Active := False;
  Table1.Exclusive := False;
  Table1.Active := True;
end;

Since the THeaderControl will be taking the place of the Grid's column headers, we first remove (set to False) the dgTitles option in the DBGrid's Options property. Then, we add a column to the HeaderControl and set its width to 12. This will be a blank column that is the same width as the Grid's status area on the left. Next we need to make sure the Table is opened for Exclusive use (no other users can be using it). I will explain why in just a bit. Now we add the HeaderControl sections. For each one we add, we will be giving it the same text as the caption of that column in the DBGrid. We loop through the DBGrid columns, and for each one we copy over the column's caption and width. We also set the HeaderControl's MinWidth and MaxWidth properties to the same as the column width. This will prevent the column from being resized. If you need resizeable columns, you will need a bit more code, and I wanted to keep this short and sweet.

Now comes the interesting part. We are going to create an index for each column in the DBGrid. The name of the index will be the same as the columns title. This step is in a try..finally structure because there are some fields that cannot be indexed (Blobs & Memos for example). When it tries to index on these fields, it will generate an exception. We catch this exception and turn off the ability to click that column. This means that non-indexed columns will not respond to mouse clicks. The creation of these indexes is why we had to open the table in Exclusive mode. After we are all done, we close the table, set Exclusive off and reopen then table. One last step. When the HeaderControl is clicked, we need to turn on the correct index for the Table. The HeaderControl's OnSectionClick method should be as follows:

procedure TForm1.HeaderControl1SectionClick(HeaderControl: THeaderControl;
  Section: THeaderSection);
begin
  Table1.IndexName := Section.Text;
end;

That's it! When the column is clicked, the Table's IndexName property is set to the same as the HeaderControl's caption. There is a lot of room for improvement however. It would be nice if clicking on a column a second time would reverse the sort order. Also, column resizing would be a nice added touch.


Improvements:

The improvement over the previous version is in the usage of the fieldname as indexname instead of the caption. This improves the flexibility.

procedure TfrmDoc.FormCreate(Sender: TObject);
var
  TheCap, TheFn: string;
  TheWidth, a: Integer;
begin
  Dbgrid1.Options := DBGrid1.Options - [DGTitles];
  Headercontrol1.sections.Add;
  Headercontrol1.Sections.Items[0].Width := 12;
  for a := 1 to DBGRID1.Columns.Count do
  begin
    with DBGrid1.Columns.Items[a - 1] do
    begin
      TheFn := FieldName;
      TheCap := Title.Caption;
      TheWidth := Width;
    end;
    with Headercontrol1.Sections do
    begin
      Add;
      Items[a].Text := TheCap;
      Items[a].Width := TheWidth + 1;
      Items[a].MinWidth := TheWidth + 1;
      Items[a].MaxWidth := TheWidth + 1;
    end;
    try
      {Use indexes with the same name as the fieldname}
      (DataSource1.Dataset as TTable).IndexName := TheFn;
      {Try to set the index name}
    except
      HeaderControl1.Sections.Items[a].AllowClick := False;
      {Index not Available}
    end;
  end;
end;

Use the fieldname property of the DBGrid to set an index with the same name as the fieldname.

procedure TfrmDoc.HeaderControl1SectionClick(HeaderControl: THeaderControl;
  Section: THeaderSection);
begin
  (DataSource1.Dataset as TTable).IndexName :=
    DBGrid1.Columns.Items[Section.Index - 1].FieldName;
end;