Example 4: Applying the sort to the dataset

Uses OnSortChange to apply the order a title click asks for. The grid shows the sort direction in the title, but never reorders the data itself: the dataset does, as it knows how.

Scenario

A click on a title orders the products by that column, ascending, then descending on the next click.

Units

Besides EgFireGrid: System.StrUtils (IfThen, in the second snippet) and the unit of the dataset component (here FireDAC.Comp.Client, for Query1).

The handler

For a FireDAC dataset, IndexFieldNames orders the records in memory, with no new query:

procedure TForm1.Grid1SortChange(Sender: TObject; Column: TEgFireGridColumn; Direction: TEgFireGridSortDirection);
begin
  case Direction of
    fgsAscending:
      Query1.IndexFieldNames := Column.FieldName;
    fgsDescending:
      Query1.IndexFieldNames := Column.FieldName + ':D';
  else
    Query1.IndexFieldNames := '';
  end;
end;

For a large table, the order can instead be given to the server, with a new query:

Query1.Close;
Query1.SQL.Text := 'SELECT * FROM Products ORDER BY ' + Column.FieldName +
  IfThen(Direction = fgsDescending, ' DESC', '');
Query1.Open;

Why it works this way

• One sorted column: a click on a title clears the direction of the other columns, so IndexFieldNames names one field.
• Columns that cannot be sorted: IndexFieldNames cannot order by a lookup or calculated field, and ordering by a memo or picture makes little sense. Set those columns’ Sortable to fscNone: their title click then fires OnTitleClick only.
• A column with no field has nothing to order by: give it fscNone too.
• Reopening the query rebuilds the default columns (when Columns is empty); the grid keeps the sort glyph on the column of the same field. With the second form, build the ORDER BY from a list of allowed field names rather than from any text, as a field name is put into SQL.

See Also

• OnSortChange
• Sortable
• SortColumn
• Examples