17.5.2 設(shè)計(jì)一個(gè)數(shù)據(jù)庫查詢器

　　例17.2：在數(shù)據(jù)庫查詢器中，用戶可以選擇要查詢的數(shù)據(jù)庫，查詢數(shù)據(jù)庫中的那一個(gè)表、根據(jù)數(shù)據(jù)庫表中那一個(gè)字段進(jìn)行查詢，并且可以方便地指定查詢條件，指定查詢條件主要包括指定邏輯運(yùn)算符(=、>、<、<=、>=、like、in、NOT like、NOT in)和字段值。

　　例子全部的程序清單如下：

　　unit main;

　　interface

　　uses

　　SysUtils, Windows, Messages, Classes, Graphics, Controls,

　　Forms, Dialogs, StdCtrls, DB, DBTables, Buttons, ComCtrls, Tabnotbk;

　　type

　　TQueryForm = class(TForm)

　　BitBtn1: TBitBtn;

　　DataSource1: TDataSource;

　　Table1: TTable;

　　GroupBox1: TGroupBox;

　　CheckBox1: TCheckBox;

　　CheckBox2: TCheckBox;

　　PageControl1: TPageControl;

　　TabSheet1: TTabSheet;

　　Label5: TLabel;

　　Label1: TLabel;

　　Label2: TLabel;

　　Label3: TLabel;

　　Label4: TLabel;

　　ListBox1: TListBox;

　　ListBox2: TListBox;

　　ListBox3: TListBox;

　　Edit1: TEdit;

　　ComboBox1: TComboBox;

　　BitBtn2: TBitBtn;

　　TabSheet2: TTabSheet;

　　Memo1: TMemo;

　　procedure FormCreate(Sender: TObject);

　　procedure ListBox1Click(Sender: TObject);

　　procedure ListBox2Click(Sender: TObject);

　　procedure BitBtn2Click(Sender: TObject);

　　end;

　　var

　　QueryForm: TQueryForm;

　　implementation

　　{$R *.DFM}

　　uses RSLTFORM;

　　procedure TQueryForm.FormCreate(Sender: TObject);

　　begin

　　Screen.Cursor := crHourglass;

　　{ Populate the alias list }

　　with ListBox1 do

　　begin

　　Items.Clear;

　　Session.GetAliasNames(Items);

　　end;

　　{ Make sure there are aliases defined }

　　Screen.Cursor := crDefault;

　　if ListBox1.Items.Count < 1 then

　　MessageDlg( 'There are no database aliases currently defined. You

　　need at least one alias to use this demonstration.',

　　mtError, [mbOK], 0 );

　　{ Default the drop-down list to the first value in the list }

　　ComboBox1.ItemIndex := 0;

　　end;

　　procedure TQueryForm.ListBox1Click(Sender: TObject);

　　var

　　strValue: string; { Holds the alias selected by the user }

　　bIsLocal: Boolean; { Indicates whether or not an alias is local }

　　slParams: TStringList; { Holds the parameters of the selected alias }

　　iCounter: Integer; { An integer counter variable for loops}

　　begin

　　{ Determine the alias name selected by the user }

　　with ListBox1 do

　　strValue := Items.Strings[ItemIndex];

　　{ Get the names of the tables in the alias and put them in the

　　appropriate list box, making sure the user's choices are reflected

　　in the list. }

　　ListBox2.Items.Clear;

　　Session.GetTableNames(strValue, { alias to enumerate }

　　'', { pattern to match }

　　CheckBox1.Checked, { show extensions flag }

　　CheckBox2.Checked, { show system tables flag }

　　ListBox2.Items); { target for table list }

　　{ Make sure there are tables defined in the alias. If not, show an

　　error; otherwise, clear the list box. }

　　Screen.Cursor := crDefault;

　　if ListBox2.Items.Count < 1 then

　　MessageDlg('There are no tables in the alias you selected. Please

　　choose another', mtError, [mbOK], 0 );

　　ListBox3.Items.Clear;

　　end;

　　procedure TQueryForm.ListBox2Click(Sender: TObject);

　　begin

　　Screen.Cursor := crHourglass;

　　try

　　{ First, disable the TTable object. }

　　if Table1.Active then

　　Table1.Close;

　　{ Open the selected table }

　　with ListBox1 do

　　Table1.DatabaseName := Items.Strings[ItemIndex];

　　with ListBox2 do

　　Table1.TableName := Items.Strings[ItemIndex];

　　{ Open the table and put a list of the field names in the Fields

　　list box. }

　　Table1.Open;

　　if Table1.Active then

　　Table1.GetFieldNames(ListBox3.Items);

　　finally

　　Screen.Cursor := crDefault;

　　end;

　　end;

　　procedure TQueryForm.BitBtn2Click(Sender: TObject);

　　var

　　strAlias, { Alias name selected by the user }

　　strTable, { Table name selected by the user }

　　strField, { Field name selected by the user }

　　strValue, { Field Value entered by the user }

　　strWhere, { WHERE clause for the user's query }

　　strQuote, { Holds quotes is the query field is text }

　　strQuery: string; { String used to construct the query }

　　frmQuery: TResultForm; { The Results form }

　　type

　　{ The following type is used with the Type drop-down

　　list. The text values corresponding with each item is

　　described in comments, along with the relevant SQL operators. }

　　etSQLOps = (soNoCondition, { not field conditions: no WHERE clause }

　　soEqual, { equals: = }

　　soNotEqual, { is not equal to: <> }

　　soLessThan, { is less than: < }

　　soLessEqual, { is less than or equal to: <= }

　　soMoreThan, { is greater than: > }

　　soMoreEqual, { is greater than or equal to: >= }

　　soStartsWith, { starts with: LIKE xx% }

　　soNoStartsWith, { doesn't start with: NOT LIKE xx% }

　　soEndsWith, { ends with: LIKE %xx }

　　soNoEndsWith, { doesn't end with: NOT LIKE %xx }

　　soContains, { contains: LIKE %xx% }

　　soNoContains, { doesn't contain: NOT LIKE %xx% }

　　soBlank, { is blank: }

　　soNotBlank, { is not blank: }

　　soInside, { contains only: IN ( xx, yy, zz ) }

　　soOutside); { doesn't contain: NOT IN (xx, yy, zz) }

　　begin

　　{ Initialize the variables needed to run the query }

　　with ListBox1 do

　　if ItemIndex = -1 then

　　raise Exception.Create('Can''t Run Query: No Alias Selected')

　　else

　　strAlias := Items.Strings[ItemIndex];

　　with ListBox2 do

　　if ItemIndex = -1 then

　　raise Exception.Create('Can''t Run Query: No Table Selected')

　　else

　　strTable := Items.Strings[ItemIndex];

　　with ListBox3 do

　　if ItemIndex = -1 then

　　begin

　　if ComboBox1.ItemIndex > Ord(soNocondition) then

　　raise Exception.Create('Can''t Run Query: No Field Selected')

　　else

　　strField := '';

　　end

　　else

　　strField := Items.Strings[ItemIndex];

　　if (Edit1.Text = '') and

　　(ComboBox1.ItemIndex > Ord(soNoCondition)) and

　　(ComboBox1.ItemIndex < Ord(soBlank)) then

　　raise Exception.create('Can''t Run Query: No Search Value Entered')

　　else

　　strValue := Edit1.Text;

　　{ See if the field being search is a string field. If so, then pad the

　　quote string with quotation marks; otherwise, set it to a null value. }

　　if strField <> '' then

　　with Table1.FieldByName(strField) do

　　if (DataType = ftString) or (DataType = ftMemo) then

　　strQuote := '"' else

　　strQuote := '';

　　{ Construct the WHERE clause of the query based on the user's choice

　　in Type. }

　　case etSQLOps(ComboBox1.ItemIndex) of

　　soNoCondition: strWhere := '';

　　soEqual: strWhere := strField + ' = ' + strQuote + strValue+ strQuote;

　　soNotEqual: strWhere := strField + ' <> ' + strQuote + strValue +

　　strQuote;

　　soLessThan: strWhere := strField + ' < ' + strQuote + strValue +

　　strQuote;

　　soLessEqual: strWhere := strField + ' <= ' + strQuote + strValue +

　　strQuote;

　　soMoreThan: strWhere := strField + ' > ' + strQuote + strValue +

　　strQuote;

　　soMoreEqual: strWhere := strField + ' >= ' + strQuote + strValue +

　　strQuote;

　　soStartsWith: strWhere := strField + ' LIKE ' + strQuote +

　　strValue + '%' + strQuote;

　　soNoStartsWith: strWhere := strField + ' NOT LIKE ' + strQuote +

　　strValue + '%' + strQuote;

　　soEndsWith: strWhere := strField + ' LIKE ' + strQuote +

　　'%' + strValue + strQuote;

　　soNoEndsWith: strWhere := strField + ' NOT LIKE ' +

　　strQuote + '%' + strValue + strQuote;

　　soContains: strWhere := strField + ' LIKE '+ strQuote+'%'+ strValue

　　+ '%' + strQuote;

　　soNoContains: strWhere := strField + ' NOT LIKE ' + strQuote + '%'

　　+ strValue + '%' + strQuote;

　　soBlank: strWhere := strField + ' IS NULL';

　　soNotBlank: strWhere := strField + ' IS NOT NULL';

　　end;

　　if ComboBox1.ItemIndex = Ord(soNoCondition) then

　　strQuery := 'SELECT * FROM "' + strTable + '"'

　　else if Table1.FieldByName(strField).DataType = ftString then

　　strQuery := 'SELECT * FROM "' + strTable + '" t WHERE t.' + strWhere

　　else

　　strQuery := 'SELECT * FROM "' + strTable + '" t WHERE t.' + strWhere;

　　{ Create an instance of the browser form. }

　　frmQuery := TResultForm.Create(Application);

　　{ Use a resource protection block in case an exception is raised. This

　　ensures that the memory allocated for the Results form is released. }

　　try

　　with frmQuery do

　　begin

　　Screen.Cursor := crHourglass;

　　if Query1.Active then Query1.Close;

　　Query1.DatabaseName := strAlias; {set the alias the query poitns to}

　　Query1.SQL.clear; { empty existing SQL in the query }

　　Query1.SQL.Add(strQuery); { add query string to query object }

　　Query1.Active := True; { try to run the query }

　　Screen.Cursor := crDefault;

　　if Query1.Active then

　　begin

　　{ If the query didn't return any records, there's no point in

　　displaying the form. In that event, raise an exception. }

　　if Query1.RecordCount < 1 then

　　raise Exception.create('No records matched your criteria.

　　Please try again.' );

　　{ write a message to the browse form's status line }

　　if strField = '' then

　　Panel3.Caption := 'Now showing all records from ' + strTable

　　+ '...'

　　else

　　Panel3.Caption := 'Now showing '+ strTable +' where '+ strField

　　+' contains values equal to '+ strValue + '...';

　　{ show the form }

　　ShowModal;

　　end;

　　end;

　　finally

　　frmQuery.Free;

　　end;

　　end;

　　end.

　　unit RSLTFORM;

　　interface

　　uses

　　SysUtils, Windows, Messages, Classes, Graphics, Controls, StdCtrls, DB,

　　Forms, DBCtrls, DBGrids, DBTables, Buttons, Grids, ExtCtrls, Dialogs;

　　type

　　TResultForm = class(TForm)

　　DBGrid1: TDBGrid;

　　DBNavigator: TDBNavigator;

　　Panel1: TPanel;

　　DataSource1: TDataSource;

　　Panel2: TPanel;

　　Panel3: TPanel;

　　Query1: TQuery;

　　SpeedButton2: TSpeedButton;

　　Panel4: TPanel;

　　SpeedButton1: TSpeedButton;

　　procedure SpeedButton1Click(Sender: TObject);

　　procedure SpeedButton2Click(Sender: TObject);

　　end;

　　var

　　ResultForm: TResultForm;

　　implementation

　　{$R *.DFM}

　　procedure TResultForm.SpeedButton1Click(Sender: TObject);

　　begin

　　Close;

　　end;

　　procedure TResultForm.SpeedButton2Click(Sender: TObject);

　　var

　　strText: string; { Variable to hold display text }

　　iCounter: Integer; { Loop counter variable }

　　begin

　　{ Build a string containing the query }

　　strText := '';

　　for iCounter := 0 to Query1.SQL.Count - 1 do

　　strText := strText + Query1.SQL[iCounter];

　　{ Display the query text }

　　MessageDlg('The underlying query is: ' + #10 + #10 + strText,

　　mtInformation, [mbOK], 0 );

　　end;

　　end.