【转】Delphi多线程学习（9）：多线程数据库查询（ADO）

原文：http://www.cnblogs.com/djcsch2001/articles/2382559.html

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。

  {主窗体代码}  

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.

程序运行结果如下：

可以看到三个线程同时执行。第一第三两个线程条件一样，查询的结果也一样。