Delphi多线程学习，9：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}

1. unit Main;
2. interface
3. uses
4. Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
5. 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
23. { Private declarations }
24. public
25. { Public declarations }
26. end;
27. var
28. Form2: TForm2;
29. implementation
30. uses ADOThread;
31. {$R *.dfm}
32. procedure TForm2.Button1Click(Sender: TObject);
33. const
34. SQL_CONST='Select SaleDate from orders where CustNo = %d';
35. var
36. c1,c2,c3:Integer;
37. s1,s2,s3:string;
38. begin
39. //取得三个选择框客户的编码
40. c1:=Integer(ComboBox1.Items.Objects[ComboBox1.ItemIndex]);
41. c2:=Integer(ComboBox2.Items.Objects[ComboBox2.ItemIndex]);
42. c3:=Integer(ComboBox3.Items.Objects[ComboBox3.ItemIndex]);
43. //生成SQL 查询语句
44. s1:=Format(SQL_CONST,[c1]);
45. s2:=Format(SQL_CONST,[c2]);
46. s3:=Format(SQL_CONST,[c3]);
47. //三个线程同时查询
48. TADOThread.Create(s1,ListBox1,Label1);
49. TADOThread.Create(s2,ListBox2,Label2);
50. TADOThread.Create(s3,ListBox3,Label3);
51. end;
52. procedure TForm2.FormCreate(Sender: TObject);
53. var
54. strSQL:string;
55. begin
56. strSQL:='SELECT CustNo,Company FROM customer';
57. ADOQuery1.Close;
58. ADOQuery1.SQL.Clear;
59. ADOQuery1.SQL.Add(strSQL);
60. ADOQuery1.Open;
61. ComboBox1.Clear;
62. ComboBox2.Clear;
63. ComboBox3.Clear;
64. //将客户Company和相关CustNo填到ComboBox中
65. while not ADOQuery1.Eof do
66. begin
67. ComboBox1.AddItem(ADOQuery1.Fields[1].asString,
68. TObject(ADOQuery1.Fields[0].AsInteger));
69. ADOQuery1.Next;
70. end;
71. ComboBox2.Items.Assign(ComboBox1.Items);
72. ComboBox3.Items.Assign(ComboBox1.Items);
73. // 默认选中第一个
74. ComboBox1.ItemIndex := 0;
75. ComboBox2.ItemIndex := 0;
76. ComboBox3.ItemIndex := 0;
77. end;
78. end.{ADO查询多线程单元}
79. unit ADOThread;
80. interface
81. uses
82. Classes,StdCtrls,ADODB;
83. type
84. TADOThread = class(TThread)
85. private
86. { Private declarations }
87. FListBox:TListBox;
88. FLabel:TLabel;
89. ConnString:WideString;
90. FSQLString:string;
91. procedure UpdateCount;
92. protected
93. procedure Execute; override;
94. public
95. constructor Create(SQL:string;LB:TListBox;Lab:TLabel);
96. end;
97. implementation
98. uses Main,SysUtils,ActiveX;
99. { TADOThread }
100. constructor TADOThread.Create(SQL: string; LB: TListBox;Lab:TLabel);
101. begin
102. ConnString:=Form2.ADOConnection1.ConnectionString;
103. FListBox:=LB;
104. FLabel:=Lab;
105. FSQLString:=SQL;
106. Inherited Create(False);
107. end;
108. procedure TADOThread.Execute;
109. var
110. Qry:TADOQuery;
111. i:Integer;
112. begin
113. { Place thread code here }
114. FreeOnTerminate:=True;
115. CoInitialize(nil); //必须调用（需Uses ActiveX）
116. Qry:=TADOQuery.Create(nil);
117. try
118. Qry.ConnectionString:=ConnString; //必须有自己的连接
119. Qry.Close;
120. Qry.SQL.Clear;
121. Qry.SQL.Add(FSQLString);
122. Qry.Open;
123. FListBox.Clear;
124. for i := 0 to 100 do //为了执行久点重复历遍数据集101次
125. begin
126. while not Qry.Eof And not Terminated do
127. begin
128. FListBox.AddItem(Qry.Fields[0].asstring,nil);
129. //如果不调用Synchronize，会出现Canvas Does NOT Allow Drawing
130. Synchronize(UpdateCount);
131. Qry.Next;
132. end;
133. Qry.First;
134. FListBox.AddItem('*******',nil);
135. end;
136. finally
137. Qry.Free;
138. end;
139. CoUninitialize;
140. end;
141. procedure TADOThread.UpdateCount;
142. begin
143. FLabel.Caption:=IntToStr(FListBox.Items.Count);
144. end;
145. end.

程序运行结果如下：

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。