procedure Tfrm_yjb.ShowGrid;
Var
S: String;
Const
MySQL = 'select CardID, stuffname, BrchName, CardTime, CASE kqFlag WHEN 0 THEN ''正常'' WHEN 1 THEN ''迟到'' '
+ 'WHEN 2 THEN ''早退'' END FROM kqview WHERE CardTime >= ''%S'' AND CardTime <= ''%S''';
begin
S := Format(MySQL, [FormatDateTime('yyyy-mm-dd', dtFrom.Date), FormatDateTime('yyyy-mm-dd', dtTo.Date)]); //这句是什么意思啊？？？
ChangeDatasource(S, 0);
end;

格式化字符串，
MySQL = 'select CardID, stuffname, BrchName, CardTime, CASE kqFlag WHEN 0 THEN ''正常'' WHEN 1 THEN ''迟到'' '
+ 'WHEN 2 THEN ''早退'' END FROM kqview WHERE CardTime >= ''%S'' AND CardTime <= ''%S''';
第一个%s:dtFrom.Date
第二个%s：dtTo.Date
FormatDateTime 同理是格式化日期类型