Delphi多线程数据库查询（ADO）

Delphi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。

{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。
http://blog.csdn.net/tercel99/article/details/5783593