学习 SQL 语句 Select(7): 分组统计之 Avg()、Sum()、Max()、Min()、Count()

Group By

本来 Group By 也可以根据多个字段分组("," 隔开), 但这个例子只有 Continent 字段适合分组,
也只有 Area、Population 字段适合统计.

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本例效果图:

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代码文件:

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unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ExtCtrls, Grids, DBGrids, DB, ADODB;

type
  TForm1 = class(TForm)
    DBGrid1: TDBGrid;
    DataSource1: TDataSource;
    ADODataSet1: TADODataSet;
    Panel1: TPanel;
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    Button4: TButton;
    Button5: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure Button4Click(Sender: TObject);
    procedure Button5Click(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  with ADODataSet1 do begin
    Close;
    CommandText := 'SELECT Continent, ' +
                   'AVG(Area) AS 平均面积, AVG(Population) AS 平均人口 ' +
                   'FROM country GROUP BY Continent';
    Open;
  end;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  with ADODataSet1 do begin
    Close;
    CommandText := 'SELECT Continent, ' +
                   'SUM(Area) AS 面积总数, SUM(Population) AS 人口总数 ' +
                   'FROM country GROUP BY Continent';
    Open;
  end;
end;

procedure TForm1.Button3Click(Sender: TObject);
begin
  with ADODataSet1 do begin
    Close;
    CommandText := 'SELECT Continent, ' +
                   'MAX(Area) AS 面积最大, MIN(Population) AS 人口最少 ' +
                   'FROM country GROUP BY Continent';
    Open;
  end;
end;

procedure TForm1.Button4Click(Sender: TObject);
begin
  with ADODataSet1 do begin
    Close;
    CommandText := 'SELECT Continent, ' +
                   'MAX(Area + Population) AS 面积与人口总和最多, ' +
                   'MIN(Area + Population) AS 面积与人口总和最少 ' +
                   'FROM country GROUP BY Continent';
    Open;
  end;
end;

procedure TForm1.Button5Click(Sender: TObject);
begin
  with ADODataSet1 do begin
    Close;
    CommandText := 'SELECT COUNT(*) AS 记录总数 FROM country';
    Open;
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  mdbFile: string;
begin
  mdbFile := GetEnvironmentVariable('COMMONPROGRAMFILES');
  mdbFile := mdbFile + '\CodeGear Shared\Data\dbdemos.mdb';

  ADODataSet1.ConnectionString := 'Provider=Microsoft.Jet.OLEDB.4.0;Data Source=' +
    mdbFile + ';Persist Security Info=False';

  DBGrid1.DataSource := DataSource1;
  DataSource1.DataSet := ADODataSet1;
end;

end.

窗体文件:

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object Form1: TForm1
  Left = 0
  Top = 0
  Caption = 'Form1'
  ClientHeight = 407
  ClientWidth = 626
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  OnCreate = FormCreate
  PixelsPerInch = 96
  TextHeight = 13
  object DBGrid1: TDBGrid
    Left = 0
    Top = 33
    Width = 626
    Height = 374
    Align = alClient
    DataSource = DataSource1
    TabOrder = 0
    TitleFont.Charset = DEFAULT_CHARSET
    TitleFont.Color = clWindowText
    TitleFont.Height = -11
    TitleFont.Name = 'Tahoma'
    TitleFont.Style = []
  end
  object Panel1: TPanel
    Left = 0
    Top = 0
    Width = 626
    Height = 33
    Align = alTop
    Caption = 'Panel1'
    TabOrder = 1
    object Button1: TButton
      Left = 6
      Top = 5
      Width = 75
      Height = 25
      Caption = 'Button1'
      TabOrder = 0
      OnClick = Button1Click
    end
    object Button2: TButton
      Left = 87
      Top = 5
      Width = 75
      Height = 25
      Caption = 'Button2'
      TabOrder = 1
      OnClick = Button2Click
    end
    object Button3: TButton
      Left = 168
      Top = 5
      Width = 75
      Height = 25
      Caption = 'Button3'
      TabOrder = 2
      OnClick = Button3Click
    end
    object Button4: TButton
      Left = 249
      Top = 5
      Width = 75
      Height = 25
      Caption = 'Button4'
      TabOrder = 3
      OnClick = Button4Click
    end
    object Button5: TButton
      Left = 330
      Top = 5
      Width = 75
      Height = 25
      Caption = 'Button5'
      TabOrder = 4
      OnClick = Button5Click
    end
  end
  object DataSource1: TDataSource
    DataSet = ADODataSet1
    Left = 184
    Top = 112
  end
  object ADODataSet1: TADODataSet
    CursorType = ctStatic
    Parameters = <>
    Left = 232
    Top = 184
  end
end