[delphi技术]Delphi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。

 1 {主窗体代码}
 2 unit Main;
 3 interface
 4   uses Windows, Messages, SysUtils, Variants,
 5     Classes, Graphics, Controls, Forms, Dialogs, DB, ADODB, StdCtrls;
 6 type
 7   TForm2 = class(TForm)
 8   ComboBox1: TComboBox;
 9   ComboBox2: TComboBox;
10   ComboBox3: TComboBox;
11   ListBox1: TListBox;
12   ListBox2: TListBox;
13   ListBox3: TListBox;
14   Button1: TButton;
15   ADOConnection1: TADOConnection;
16   ADOQuery1: TADOQuery;
17   Label1: TLabel;
18   Label2: TLabel;
19   Label3: TLabel;
20   procedure FormCreate(Sender: TObject);
21   procedure Button1Click(Sender: TObject);
22   private { Private declarations }
23   public { Public declarations }
24   end;
25 var Form2: TForm2;
26 implementation
27 uses
28   ADOThread;
29   {$R *.dfm}
30
31 procedure TForm2.Button1Click(Sender: TObject);
32 const
33   SQL_CONST=‘Select SaleDate from orders where CustNo = %d‘;
34   var c1,c2,c3:Integer; s1,s2,s3:string;
35 begin //取得三个选择框客户的编码
36   c1:=Integer(ComboBox1.Items.Objects[ComboBox1.ItemIndex]);
37   c2:=Integer(ComboBox2.Items.Objects[ComboBox2.ItemIndex]);
38   c3:=Integer(ComboBox3.Items.Objects[ComboBox3.ItemIndex]); //生成SQL 查询语句
39   s1:=Format(SQL_CONST,[c1]);
40   s2:=Format(SQL_CONST,[c2]);
41   s3:=Format(SQL_CONST,[c3]); //三个线程同时查询
42   TADOThread.Create(s1,ListBox1,Label1);
43   TADOThread.Create(s2,ListBox2,Label2);
44   TADOThread.Create(s3,ListBox3,Label3);
45 end;
46
47 procedure TForm2.FormCreate(Sender: TObject);
48 var
49   strSQL:string;
50 begin
51   strSQL:=‘SELECT CustNo,Company FROM customer‘;
52   ADOQuery1.Close;
53   ADOQuery1.SQL.Clear;
54   ADOQuery1.SQL.Add(strSQL);
55   ADOQuery1.Open;
56   ComboBox1.Clear;
57   ComboBox2.Clear;
58   ComboBox3.Clear; //将客户Company和相关CustNo填到ComboBox中
59   while not ADOQuery1.Eof do
60   begin
61     ComboBox1.AddItem(ADOQuery1.Fields[1].asString, TObject(ADOQuery1.Fields[0].AsInteger));
62     ADOQuery1.Next;
63   end;
64   ComboBox2.Items.Assign(ComboBox1.Items);
65   ComboBox3.Items.Assign(ComboBox1.Items); // 默认选中第一个
66   ComboBox1.ItemIndex := 0;
67   ComboBox2.ItemIndex := 0;
68   ComboBox3.ItemIndex := 0;
69 end;
70 end.

 1 {ADO查询多线程单元}
 2 unit ADOThread;
 3 interface
 4 uses
 5   Classes,StdCtrls,ADODB;
 6 type TADOThread = class(TThread)
 7   private { Private declarations }
 8   FListBox:TListBox;
 9   FLabel:TLabel;
10   ConnString:WideString;
11   FSQLString:string;
12   procedure UpdateCount;
13   protected procedure Execute; override;
14   public constructor Create(SQL:string;LB:TListBox;Lab:TLabel);
15 end;
16
17 implementation
18 uses
19   Main,SysUtils,ActiveX;
20   { TADOThread }
21
22 constructor TADOThread.Create(SQL: string; LB: TListBox;Lab:TLabel);
23 begin
24   ConnString:=Form2.ADOConnection1.ConnectionString;
25   FListBox:=LB;
26   FLabel:=Lab;
27   FSQLString:=SQL;
28   Inherited Create(False);
29 end;
30
31 procedure TADOThread.Execute;
32 var
33   Qry:TADOQuery;
34   i:Integer;
35 begin { Place thread code here }
36   FreeOnTerminate:=True;
37   CoInitialize(nil); //必须调用（需Uses ActiveX）
38   Qry:=TADOQuery.Create(nil);
39   try
40     Qry.ConnectionString:=ConnString; //必须有自己的连接
41     Qry.Close;
42     Qry.SQL.Clear;
43     Qry.SQL.Add(FSQLString);
44     Qry.Open;
45     FListBox.Clear;
46     for i := 0 to 100 do //为了执行久点重复历遍数据集101次
47     begin
48       while not Qry.Eof And not Terminated do
49       begin
50         FListBox.AddItem(Qry.Fields[0].asstring,nil); //如果不调用Synchronize，会出现Canvas Does NOT Allow Drawing
51         Synchronize(UpdateCount);
52         Qry.Next;
53       end;
54       Qry.First;
55       FListBox.AddItem(‘*******‘,nil);
56     end;
57   finally
58     Qry.Free;
59   end;
60   CoUninitialize;
61 end;
62
63 procedure TADOThread.UpdateCount;
64 begin
65   FLabel.Caption:=IntToStr(FListBox.Items.Count);
66 end;
67 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。