ADO多線程數據庫查詢

ADO多線程數據庫查詢通常會出現3個問題：1、CoInitialize 沒有調用 （CoInitialize was not called）；所以，在使用任何dbGo對象前，必須手 調用CoInitialize和CoUninitialize。調用CoInitialize失敗會產生"CoInitialize was not called"例外。2、畫布不允許繪畫 （Canvas does not allow drawing）；所以，必須通過Synchronize過程來通知主線程訪問主窗體上的任何控件。3、不能使用主ADO連接 （Main TADoConnection cannot be used!）；所以，線程中不能使用主線程中TADOConnection對象，每個線程必須創建自己的數據庫連接。Delphi2007安裝后在X:/Program Files/Common Files/CodeGear Shared/Data目錄下有一個dbdemos.mdb文件，用來作為測試的例子。dbdemos.mdb中的customer表保存了客戶信 息，orders表中保存了訂單信息。測試程序流程大致是這樣的：在主窗體上放TADOConnection和TQuery控件，啟動時這個TQuery從Customer表中查出客戶編碼 CustNo和公司名稱Company，放到三個Combox框中，分別在三個列表框中選定客戶公司名稱，按照公司名稱所對應的客戶代碼建立三個線程同時 在orders表中查詢銷售日期SaleDate分別填入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//取得三個選擇框客戶的編碼
c1:= Integer(ComboBox1.Items.Objects[ComboBox1.ItemIndex]);c2:= Integer(ComboBox2.Items.Objects[ComboBox2.ItemIndex]);c3:= Integer(ComboBox3.Items.Objects[ComboBox3.ItemIndex]);//生成SQL 查詢語句s1:=Format(SQL_CONST,[c1]);s2:=Format(SQL_CONST,[c2]);s3:=Format(SQL_CONST,[c3]);//三個線程同時查詢
  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;//將客戶Company和相關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);// 默認選中第一個ComboBox1.ItemIndex := 0;ComboBox2.ItemIndex := 0;ComboBox3.ItemIndex := 0;
end ;end.
{ADO查詢多線程單元}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);//必須調用（需Uses ActiveX）Qry:=TADOQuery.Create(nil);tryQry.ConnectionString:=ConnString;   //必須有自己的連接
    Qry.Close;Qry.SQL.Clear;Qry.SQL.Add(FSQLString);Qry.Open;FListBox.Clear;for  i := 0 to 100 do     //為了執行久點重復歷遍數據集101次beginwhile  not Qry.Eof And  not Terminated dobeginFListBox.AddItem(Qry.Fields[0].asstring,nil);//如果不調用Synchronize，會出現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.

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