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Client Dataset Basics

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This chapter is from the book

This chapter is from the book

Searching

In addition to filtering out uninteresting records from a client dataset, TClientDataSet provides a number of methods for quickly locating a specific record. Some of these methods require an index to be active on the dataset, and others do not. The search methods are described in detail in the following sections.

Nonindexed Search Techniques

In this section, I'll discuss the search techniques that don't require an active index on the client dataset. Rather than using an index, these methods perform a sequential search through the dataset to find the first matching record.

Locate

Locate is perhaps the most general purpose of the TClientDataSet search methods. You can use Locate to search for a record based on any given field or combination of fields. Locate can also search for records based on a partial match, and can find a match without respect to case.

TClientDataSet.Locate is defined like this:

function Locate(const KeyFields: string; const KeyValues: Variant;
 Options: TLocateOptions): Boolean; override;

The first parameter, KeyFields, designates the field (or fields) to search. When searching multiple fields, separate them by semicolons (for example, 'Name;Birthday').

The second parameter, KeyValues, represents the values to search for. The number of values must match the number of key fields exactly. If there is only one search field, you can simply pass the value to search for here. To search for multiple values, you must pass the values as a variant array. One way to do this is by calling VarArrayOf, like this:

VarArrayOf(['John Smith', '4/15/1965'])

The final parameter, Options, is a set that determines how the search is to be executed. Table 3.11 lists the available options.

Table 3.11 Locate Options

Value

Description

loPartialKey

KeyValues do not necessarily represent an exact match. Locate finds the first record whose field value starts with the value specified in KeyValues.

loCaseInsensitive

Locate ignores case when searching for string fields.


Both options pertain to string fields only. They are ignored if you specify them for a nonstring search.

Locate returns True if a matching record is found, and False if no match is found. In case of a match, the record is made current.

The following examples help illustrate the options:

ClientDataSet1.Locate('Name', 'John Smith', []);

This searches for a record where the name is 'John Smith'.

ClientDataSet1.Locate('Name', 'JOHN', [loPartialKey, loCaseInsensitive]);

This searches for a record where the name begins with 'JOHN'. This finds 'John Smith', 'Johnny Jones', and 'JOHN ADAMS', but not 'Bill Johnson'.

ClientDataSet1.Locate('Name;Birthday', VarArrayOf(['John', '4/15/1965']), 
 [loPartialKey]);

This searches for a record where the name begins with 'John' and the birthday is April 15, 1965. In this case, the loPartialKey option applies to the name only. Even though the birthday is passed as a string, the underlying field is a date field, so the loPartialKey option is ignored for that field only.

Lookup

Lookup is similar in concept to Locate, except that it doesn't change the current record pointer. Instead, Lookup returns the values of one or more fields in the record. Also, Lookup does not accept an Options parameter, so you can't perform a lookup that is based on a partial key or that is not case sensitive.

Lookup is defined like this:

function Lookup(const KeyFields: string; const KeyValues: Variant;
 const ResultFields: string): Variant; override;

KeyFields and KeyValues specify the fields to search and the values to search for, just as with the Locate method. ResultFields specifies the fields for which you want to return data. For example, to return the birthday of the employee named John Doe, you could write the following code:

var
 V: Variant;
begin
 V := ClientDataSet1.Lookup('Name', 'John Doe', 'Birthday');
end;

The following code returns the name and birthday of the employee with ID number 100.

var
 V: Variant;
begin
 V := ClientDataSet1.Lookup('ID', 100, 'Name;Birthday');
end;

If the requested record is not found, V is set to NULL. If ResultFields contains a single field name, then on return from Lookup, V is a variant containing the value of the field listed in ResultFields. If ResultFields contains multiple single-field names, then on return from Lookup, V is a variant array containing the values of the fields listed in ResultFields.

NOTE

For a comprehensive discussion of variant arrays, see my book, Delphi COM Programming, published by Macmillan Technical Publishing.

The following code snippet shows how you can access the results that are returned from Lookup.

var
 V: Variant;
begin
 V := ClientDataSet1.Lookup('ID', 100, 'Name');
 if not VarIsNull(V) then
 ShowMessage('ID 100 refers to ' + V);

 V := ClientDataSet1.Lookup('ID', 200, 'Name;Birthday');
 if not VarIsNull(V) then
 ShowMessage('ID 200 refers to ' + V[0] + ', born on ' + DateToStr(V[1]));
end;

Indexed Search Techniques

The search techniques mentioned earlier do not require an index to be active (in fact, they don't require the dataset to be indexed at all), but TDataSet also supports several indexed search operations. These include FindKey, FindNearest, and GotoKey, which are discussed in the following sections.

FindKey

FindKey searches for an exact match on the key fields of the current index. For example, if the dataset is currently indexed by ID, FindKey searches for an exact match on the ID field. If the dataset is indexed by last and first name, FindKey searches for an exact match on both the last and the first name.

FindKey takes a single parameter, which specifies the value(s) to search for. It returns a Boolean value that indicates whether a matching record was found. If no match was found, the current record pointer is unchanged. If a matching record is found, it is made current.

The parameter to FindKey is actually an array of values, so you need to put the values in brackets, as the following examples show:

if ClientDataSet.FindKey([25]) then
 ShowMessage('Found ID 25');
...
if ClientDataSet.FindKey(['Doe', 'John']) then
 ShowMessage('Found John Doe');

You need to ensure that the values you search for match the current index. For that reason, you might want to set the index before making the call to FindKey. The following code snippet illustrates this:

ClientDataSet1.IndexName := 'byID';
if ClientDataSet.FindKey([25]) then
 ShowMessage('Found ID 25');
...
ClientDataSet1.IndexName := 'byName';
if ClientDataSet.FindKey(['Doe', 'John']) then
 ShowMessage('Found John Doe');

FindNearest

FindNearest works similarly to FindKey, except that it finds the first record that is greater than or equal to the value(s) passed to it. This depends on the current value of the KeyExclusive property.

If KeyExclusive is False (the default), FindNearest finds the first record that is greater than or equal to the passed-in values. If KeyExclusive is True, FindNearest finds the first record that is greater than the passed-in values.

If FindNearest doesn't find a matching record, it moves the current record pointer to the end of the dataset.

GotoKey

GotoKey performs the same function as FindKey, except that you set the values of the search field(s) before calling GotoKey. The following code snippet shows how to do this:

ClientDataSet1.IndexName := 'byID';
ClientDataSet1.SetKey;
ClientDataSet1.FieldByName('ID').AsInteger := 25;
ClientDataSet1.GotoKey;

If the index is made up of multiple fields, you simply set each field after the call to SetKey, like this:

ClientDataSet1.IndexName := 'byName';
ClientDataSet1.SetKey;
ClientDataSet1.FieldByName('First').AsString := 'John';
ClientDataSet1.FieldByName('Last').AsString := 'Doe';
ClientDataSet1.GotoKey;

After calling GotoKey, you can use the EditKey method to edit the key values used for the search. For example, the following code snippet shows how to search for John Doe, and then later search for John Smith. Both records have the same first name, so only the last name portion of the key needs to be specified during the second search.

ClientDataSet1.IndexName := 'byName';
ClientDataSet1.SetKey;
ClientDataSet1.FieldByName('First').AsString := 'John';
ClientDataSet1.FieldByName('Last').AsString := 'Doe';
ClientDataSet1.GotoKey;
// Do something with the record

// EditKey preserves the values set during the last SetKey
ClientDataSet1.EditKey;
ClientDataSet1.FieldByName('Last').AsString := 'Smith';
ClientDataSet1.GotoKey;

GotoNearest

GotoNearest works similarly to GotoKey, except that it finds the first record that is greater than or equal to the value(s) passed to it. This depends on the current value of the KeyExclusive property.

If KeyExclusive is False (the default), GotoNearest finds the first record that is greater than or equal to the field values set after a call to either SetKey or EditKey. If KeyExclusive is True, GotoNearest finds the first record that is greater than the field values set after calling SetKey or EditKey.

If GotoNearest doesn't find a matching record, it moves the current record pointer to the end of the dataset.

The following example shows how to perform indexed and nonindexed searches on a dataset.

Listing 3.7 shows the source code for the Search application, a sample program that illustrates the various indexed and nonindexed searching techniques supported by TClientDataSet.

unit MainForm;

interface

uses
 SysUtils, Classes, Variants, QGraphics, QControls, QForms, QDialogs,
 QStdCtrls, DB, DBClient, QExtCtrls, QActnList, QGrids, QDBGrids;

type
 TfrmMain = class(TForm)
 DataSource1: TDataSource;
 pnlClient: TPanel;
 pnlBottom: TPanel;
 btnSearch: TButton;
 btnGotoBookmark: TButton;
 btnGetBookmark: TButton;
 btnLookup: TButton;
 DBGrid1: TDBGrid;
 ClientDataSet1: TClientDataSet;
 btnSetRecNo: TButton;
 procedure FormCreate(Sender: TObject);
 procedure btnGetBookmarkClick(Sender: TObject);
 procedure btnGotoBookmarkClick(Sender: TObject);
 procedure btnSetRecNoClick(Sender: TObject);
 procedure btnSearchClick(Sender: TObject);
 procedure btnLookupClick(Sender: TObject);
 private
 { Private declarations }
 FBookmark: TBookmark;
 public
 { Public declarations }
 end;

var
 frmMain: TfrmMain;

implementation

uses SearchForm;

{$R *.xfm}

procedure TfrmMain.FormCreate(Sender: TObject);
begin
 ClientDataSet1.LoadFromFile('C:\Employee.cds');

 ClientDataSet1.AddIndex('byName', 'Name', []);
 ClientDataSet1.IndexName := 'byName';
end;

procedure TfrmMain.btnGetBookmarkClick(Sender: TObject);
begin
 if Assigned(FBookmark) then
 ClientDataSet1.FreeBookmark(FBookmark);

 FBookmark := ClientDataSet1.GetBookmark;
end;

procedure TfrmMain.btnGotoBookmarkClick(Sender: TObject);
begin
 if Assigned(FBookmark) then
 ClientDataSet1.GotoBookmark(FBookmark)
 else
 ShowMessage('No bookmark assigned');
end;

procedure TfrmMain.btnSetRecNoClick(Sender: TObject);
var
 Value: string;
begin
 Value := '1';
 if InputQuery('RecNo', 'Enter Record Number', Value) then
 ClientDataSet1.RecNo := StrToInt(Value);
end;

procedure TfrmMain.btnSearchClick(Sender: TObject);
var
 frmSearch: TfrmSearch;
begin
 frmSearch := TfrmSearch.Create(nil);
 try
 if frmSearch.ShowModal = mrOk then begin
  case TSearchMethod(frmSearch.grpMethod.ItemIndex) of
  smLocate:
   ClientDataSet1.Locate('Name', frmSearch.ecName.Text,
   [loPartialKey, loCaseInsensitive]);

  smFindKey:
   ClientDataSet1.FindKey([frmSearch.ecName.Text]);

  smFindNearest:
   ClientDataSet1.FindNearest([frmSearch.ecName.Text]);

  smGotoKey: begin
   ClientDataSet1.SetKey;
   ClientDataSet1.FieldByName('Name').AsString :=
   frmSearch.ecName.Text;
   ClientDataSet1.GotoKey;
  end;

  smGotoNearest: begin
   ClientDataSet1.SetKey;
   ClientDataSet1.FieldByName('Name').AsString :=
   frmSearch.ecName.Text;
   ClientDataSet1.GotoNearest;
  end;
  end;
 end;
 finally
 frmSearch.Free;
 end;
end;

procedure TfrmMain.btnLookupClick(Sender: TObject);
var
 Value: string;
 V: Variant;
begin
 Value := '1';
 if InputQuery('ID', 'Enter ID to Lookup', Value) then begin
 V := ClientDataSet1.Lookup('ID', StrToInt(Value), 'Name;Salary');
 if not VarIsNull(V) then
  ShowMessage(Format('ID %s refers to %s, who makes %s',
  [Value, V[0], FloatToStrF(V[1], ffCurrency, 10, 2)]));
 end;
end;

end.

Listing 3.8 contains the source code for the search form. The only interesting bit of code in this listing is the TSearchMethod, defined near the top of the unit, which is used to determine what method to call for the search.

Listing 3.8 Search—SearchForm.pas

unit SearchForm;

interface

uses
 SysUtils, Classes, QGraphics, QControls, QForms, QDialogs, QExtCtrls,
 QStdCtrls;

type
 TSearchMethod = (smLocate, smFindKey, smFindNearest, smGotoKey,
 smGotoNearest);

 TfrmSearch = class(TForm)
 pnlClient: TPanel;
 pnlBottom: TPanel;
 Label1: TLabel;
 ecName: TEdit;
 grpMethod: TRadioGroup;
 btnOk: TButton;
 btnCancel: TButton;
 private
 { Private declarations }
 public
 { Public declarations }
 end;

implementation

{$R *.xfm}

end.

Figure 3.9 shows the Search application at runtime.

Figure 3.9 Search demonstrates indexed and nonindexed searches.

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