ADO多线程数据库查询

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unitMain;interfaceusesWindows,  Messages,  SysUtils,   Variants, Classes, Graphics,   Controls, Forms,   Dialogs,DB,       ADODB,      StdCtrls;typeTForm2 =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    ;
varForm2:    TForm2;
implementationusesADOThread;{$R *.dfm}procedure   TForm2.Button1Click(Sender: TObject);
constSQL_CONST= 'Select SaleDate from orders where CustNo = %d' ;
varc1,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);
varstrSQL:string ;
beginstrSQL:='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  dobeginComboBox1.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}unitADOThread;interfaceusesClasses,StdCtrls,ADODB;
typeTADOThread = class(TThread)private{ Private declarations }FListBox:TListBox;FLabel:TLabel;ConnString:WideString;FSQLString:string;procedure  UpdateCount;protectedprocedure  Execute; override;publicconstructor  Create(SQL:string;LB:TListBox;Lab:TLabel);end   ;implementationuses  Main,SysUtils,ActiveX;{ TADOThread }constructor  TADOThread.Create(SQL: string; LB: TListBox;Lab:TLabel);
beginConnString:=Form2.ADOConnection1.ConnectionString;FListBox:=LB;FLabel:=Lab;FSQLString:=SQL;Inherited  Create(False);
end  ;procedure  TADOThread.Execute;
varQry:TADOQuery;i:Integer;
begin{ Place thread code here }FreeOnTerminate:=True;CoInitialize(nil);//必须调用（需Uses ActiveX）Qry:=TADOQuery.Create(nil);tryQry.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beginwhile  not Qry.Eof And  not Terminated dobeginFListBox.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 ;finallyQry.Free;end ;CoUninitialize;
end  ;procedure   TADOThread.UpdateCount;
beginFLabel.Caption:=IntToStr(FListBox.Items.Count);
end     ;end.