ADO多线程数据库查询通常会出现以下问题：
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、不能使用主线程的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。且在线程中，最好不要使用连接对象，而使用连接字符串。因为连接对对象释放时会出问题。
Delphi2007安装后在X:Program FilesCommon FilesCodeGear SharedData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。
4、COM接口调用时必须有线程自己的接口。线程的
如：在A线程建的COM接口，B线程调用时就提示“应用程序调用一个已为另一线程整理的接口”。这时只要在B线程建立自己独有的COM接口使用就可以了。
{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.
{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.