Delphi多线程学习：多线程数据库查询（ADO）

ADO多线程数据库查询通常会出现3个问题：

1、CoInitialize 没有调用 （CoInitialize was not called）；所以，在使用任何dbGo对象前，必须手 调用CoInitialize和CoUninitialize。调用CoInitialize失败会产生"CoInitialize was not called"例外。

2、画布不允许绘画 （Canvas does not allow drawing）；所以，必须通过Synchronize过程来通知主线程访问主窗体上的任何控件。

3、不能使用主ADO连接 （Main TADoConnection cannot be used!）；所以，线程中不能使用主线程中TADOConnection对象，每个线程必须创建自己的数据库连接。

     Delphi2007安装后在X:/Program Files/Common Files/CodeGear Shared/Data目录下有一个dbdemos.mdb文件，用来作为测试的例子。dbdemos.mdb中的customer表保存了客户信 息，orders表中保存了订单信息。

       测试程序流程大致是这样的：在主窗体上放TADOConnection和TQuery控件，启动时这个TQuery从Customer表中查出客户编码 CustNo和公司名称Company，放到三个Combox框中，分别在三个列表框中选定客户公司名称，按照公司名称所对应的客户代码建立三个线程同时 在orders表中查询销售日期SaleDate分别填入ListBox中

unit
 Main;

interface

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

type
  TForm2 =class (TForm)
    ComboBox1:  TComboBox;
    ComboBox2:  TComboBox;
    ComboBox3:   TComboBox;
    ListBox1:    TListBox;
    ListBox2:   TListBox;
    ListBox3:    TListBox;
    Button1:  TButton;
    ADOConnection1:  TADOConnection;
    ADOQuery1:  TADOQuery;
    Label1: TLabel;
    Label2:  TLabel;
    Label3: TLabel;

    procedure  FormCreate(Sender: TObject) ;
    procedure  Button1Click(Sender: TObject) ;
  private
    { Private declarations }
  public
    { Public declarations }
  end    ;
var
  Form2:    TForm2;
implementation

uses
 ADOThread;

{$R *.dfm}

procedure   TForm2.Button1Click(Sender: TObject);
const
  SQL_CONST= 'Select SaleDate from orders where CustNo = %d' ;
var
  c1,c2,c3: Integer  ;
  s1, s2,s3: string ;
begin

  //取得三个选择框客户的编码

  c1:= Integer(ComboBox1.Items.Objects[ComboBox1.ItemIndex]);
  c2:= Integer(ComboBox2.Items.Objects[ComboBox2.ItemIndex]);
  c3:= Integer(ComboBox3.Items.Objects[ComboBox3.ItemIndex]);
  //生成SQL 查询语句
  s1:=Format(SQL_CONST,[c1]);
  s2:=Format(SQL_CONST,[c2]);
  s3:=Format(SQL_CONST,[c3]);
  //三个线程同时查询
  TADOThread.Create(s1,ListBox1,Label1) ;
  TADOThread.Create(s2,ListBox2,Label2);
  TADOThread.Create(s3,ListBox3,Label3);
end ;

procedure TForm2.FormCreate(Sender: TObject);
var
  strSQL:string ;
begin
  strSQL:='SELECT CustNo,Company FROM customer';
  ADOQuery1.Close;
  ADOQuery1.SQL.Clear;
  ADOQuery1.SQL.Add(strSQL);
  ADOQuery1.Open;
  ComboBox1.Clear;
  ComboBox2.Clear;
  ComboBox3.Clear;
  //将客户Company和相关CustNo填到ComboBox中
  while  not  ADOQuery1.Eof  do
  begin
    ComboBox1.AddItem(ADOQuery1.Fields[1].asString, TObject(ADOQuery1.Fields[0].AsInteger));
    ADOQuery1.Next;
  end ;
  ComboBox2.Items.Assign(ComboBox1.Items);
  ComboBox3.Items.Assign(ComboBox1.Items);
  // 默认选中第一个
  ComboBox1.ItemIndex := 0;
  ComboBox2.ItemIndex := 0;
  ComboBox3.ItemIndex := 0;
end ;

end.
{ADO查询多线程单元}


unit
 ADOThread;

interface

uses
  Classes,StdCtrls,ADODB;
type
  TADOThread = class(TThread)
  private
    { Private declarations }
    FListBox:TListBox;
    FLabel:TLabel;
    ConnString:WideString;
    FSQLString:string;
    procedure  UpdateCount;
  protected
    procedure  Execute; override;
  public
    constructor  Create(SQL:string;LB:TListBox;Lab:TLabel);
  end   ;

implementation

uses  Main,SysUtils,ActiveX;

{ TADOThread }

constructor  TADOThread.Create(SQL: string; LB: TListBox;Lab:TLabel);
begin
  ConnString:=Form2.ADOConnection1.ConnectionString;
  FListBox:=LB;
    FLabel:=Lab;
  FSQLString:=SQL;
  Inherited  Create(False);
end  ;

procedure  TADOThread.Execute;
var
  Qry:TADOQuery;
  i:Integer;
begin
  { Place thread code here }
  FreeOnTerminate:=True;
  CoInitialize(nil);
    //必须调用（需Uses ActiveX）
  Qry:=TADOQuery.Create(nil);
  try
    Qry.ConnectionString:=ConnString;   //必须有自己的连接
    Qry.Close;
    Qry.SQL.Clear;
    Qry.SQL.Add(FSQLString);
    Qry.Open;
    FListBox.Clear;
    for  i := 0 to 100 do     //为了执行久点重复历遍数据集101次
      begin
        while  not Qry.Eof And  not Terminated do
        begin
        FListBox.AddItem(Qry.Fields[0].asstring,nil);
          //如果不调用Synchronize，会出现Canvas Does NOT Allow Drawing
          Synchronize(UpdateCount);
          Qry.Next;
        end ;
        Qry.First;
        FListBox.AddItem('*******',nil);
      end ;
  finally
    Qry.Free;
  end ;
  CoUninitialize;
end  ;

procedure   TADOThread.UpdateCount;
begin
  FLabel.Caption:=IntToStr(FListBox.Items.Count);
end     ;

end.