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 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.  

　　

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}