Code:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, DBXpress, DB, StdCtrls, SqlExpr, DBClient, SimpleDS, Grids,
  DBGrids;

type
  TForm1 = class(TForm)
    DBGrid1: TDBGrid;
    SimpleDataSet1: TSimpleDataSet;
    SQLConnection1: TSQLConnection;
    ComboBox1: TComboBox;
    ComboBox2: TComboBox;
    Button1: TButton;
    DataSource1: TDataSource;
    ComboBox3: TComboBox;
    SimpleDataSet2: TSimpleDataSet;
    procedure FormCreate(Sender: TObject);
    procedure ComboBox1Change(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure ComboBox2Change(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
SQLConnection1.GetTableNames(ComboBox1.Items);
end;

procedure TForm1.ComboBox1Change(Sender: TObject);
begin
SQLConnection1.GetFieldNames(ComboBox1.Text,ComboBox2.Items);
SimpleDataSet1.Close;
SimpleDataSet1.DataSet.Close;
try
  SimpleDataSet1.DataSet.CommandText:='';
  SimpleDataSet1.DataSet.CommandType:=ctTable;
  SimpleDataSet1.DataSet.CommandText:=ComboBox1.Text;
finally
  SimpleDataSet1.DataSet.Open;
  SimpleDataSet1.Open;
end;

end;

procedure TForm1.Button1Click(Sender: TObject);
begin
SimpleDataSet1.Close;
SimpleDataSet1.DataSet.Close;
try
  SimpleDataSet1.DataSet.CommandType:=ctQuery;
  SimpleDataSet1.DataSet.CommandText:='select * from '+Combobox1.text+' where '+ComboBox2.Text+' = '+QuotedStr(ComboBox3.Text);
finally
SimpleDataSet1.DataSet.Open;
SimpleDataSet1.Open;
end;

end;

procedure TForm1.ComboBox2Change(Sender: TObject);
var
i:integer;
begin
  SimpleDataSet2.Close;
  SimpleDataSet2.DataSet.Close;
  try
    SimpleDataSet2.DataSet.CommandType:=ctQuery;
    SimpleDataSet2.DataSet.CommandText:='select '+ComboBox2.Text+' from '+ComboBox1.Text;
  finally
  SimpleDataSet2.DataSet.Open;
  SimpleDataSet2.Open;
  end;
  ComboBox3.Items.BeginUpdate;
  SimpleDataSet2.First;
  try
  while not SimpleDataSet2.Eof do
    begin
      for i:=0 to SimpleDataSet2.FieldCount-1 do
      begin
        ComboBox3.Items.Add(VarToStr(SimpleDataSet2.Fields[i].Value));
        SimpleDataSet2.Next;
      end;
    end;
  finally
    ComboBox3.Items.EndUpdate;
  end;

end;

end.