Question

我正在尝试计算学校项目数据库中的所有非空字段。请注意我不允许使用SQL。

这是编码

var i,k : Integer ;
begin 
  i := 0 ;
  with dmregisteredusers do
  begin
    tblusers.Sort := 'Nommer ASC';
    tblusers.Edit;
    tblusers.First;
    For k:= 1 to tblusers.RecordCount do
    begin
      If (tblusers['Dogs wanted'] = '') OR (tblusers['Dogs wanted'] = ' ') 
        OR (tblusers['Dogswanted'] = 0) 
      then 
        tblusers.Next
      else 
      begin
        inc(i);
        tblusers.Next;
      end;//else
    end;//with
  end;//for
  ShowMessage('There are ' + IntToStr(i) + ' new dogs added to wishlist ,please contact the users regarding this matter and them remove the dogs from their wishlist !');

每次运行代码时都会显示空的Db字段为数据类型为null的错误。

如何验证数据库字段为空？

Answer 1

这是您的代码的新版本。我完成它的方式有点乱，但我认为如果我在其中包含注释和解释的话，最容易理解。

首先要说的是，虽然检查给定行的数据集字段为Null并不是坏事，但如果不允许数据库在可能被查询的列中存储Null，通常会更好。应用程序或原始Sql查询。从理论和实践的角度来看，全书的章节都是关于Nulls做什么或不做什么的。在实践中，他们通常被认为是“信息缺失”的意思。所以我们无法回答“这个用户想要一只狗吗？”的问题。在Null的存在。所以这是一个政策和设计选择的问题，如何处理Null，但通常在数据库的实现中通过对列施加NOT NULL约束来解决问题要好得多。

当我在评论中说Delphi不是特别擅长尊重Null和空白字段之间的区别时，我的意思是：在String字段的情况下，对于字段为Null的行，Delphi当您调用Field.AsString时，返回一个空字符串''。一些“纯粹主义者”会说，如果在包含Null的情况下要求其AsXXX属性，TField应该生成异常，因为它实际上不应该尝试“伪造”某个值。它的作用，即返回一个空字符串，0表示数字字段等，这是一个实用的妥协：它避免了初学者被Nulls的存在绊倒，但如果你想让你的代码处理Null，你可以使用TField.IsNull。

如果您遇到一个包含Null的数据库 - 并且您经常会感到压抑（除非您参与了数据库的设计） - 考虑一下它们最好在SQL中处理获取数据的可能性在您的Delphi代码看到它之前。

第二件事是，在没有明确的设计简报的情况下，我们并不真正知道“想要狗”的含义。这是否意味着用户想要一个单数，复数或是/否的狗？ Yes / No是最简单的处理方式，但并非所有数据库都支持显式的布尔列类型，因此您经常会看到一个（希望是单字符）CHAR，VARCHAR（或它们的Unicode等价物）列。或整数列，如果要招待挑剔的用户的要求。

第三个问题是字段的Delphi DataType不一定与db的列类型完全相同，尽管它们之间存在标准映射，无论如何，通常最好使用值表示在Delphi代码中工作（例如.AsString，.AsInteger，.AsFloat，仅举几例）与db列类型最匹配。

不可否认，并非所有的数据库实现都很繁琐，对不起，我的意思是小心，因为Delphi正在处理列的数据类型，但大多数都是。一个值得注意的例外是Sqlite，虽然您可以在表的DDL中定义列类型，但Sqlite引擎更像是推荐而不是规则，您可以在其中存储几乎任何内容。

正如我之前在评论中所说，Null是列/字段状态，而不是值。那么问“什么数据类型是一个空数据库字段？”是各种“类别错误”。列的数据类型“就像它在锡上所说的那样”，即它在表的DDL中定义的内容。当然，你真正要问的是“我如何确定该字段是否为空”，这与设计选择和数据库实现一样，与Delphi编码一样。

无论如何，足够的一般性......

procedure CountDogsWanted;
var
  // i,k : Integer ; <- names like i and k are usually as used for loop variables
  DogsWanted : Integer;
  Wanted : Boolean;
  S : String;  // contrast this naming style with what I said about the likes of i, j, k
    // I've done this because we might want to do several tests & operations on its value
    // and they will be easier to read with a shorter variable name.  Not such a good idea
    // when there are several such variables.
  AField : TField; 
const
  scDogsWanted = 'Dogs wanted';  // this is to avoid making mistakes with typos
begin

  {i := 0 ;}
  DogsWanted := 0;

  //  The point of the following line is to retrieve the field we're working with
  //  only once, rather than doing a FieldByName (which involves a serial iteration
  //  through the dataset's Fields collection) for each row in the dataset.
  //  The AField variable will remain valid for the duration of this procedure
  //  or until the dataset is closed if its Fields aren't defined as persistent ones.
  //  Persistent fields remain valid for the lifetime of their owners (usually a 
  // datamodule or form).  OTOH, "dynamic" fields are owned by the dataset and created
  // and destroyed when the dataset is opened and closed.  

  AField := dmregisteredusers.tblusers.FieldByName(scDogsWanted);

  {with dmregisteredusers do  <- Don't use "with", it only ever causes problems}
  {begin}

    {tblusers.Sort := 'Nommer ASC'; <- pointless, it makes no difference when you count what order the things are in}

    {tblusers.Edit; <- No! This puts the table into Edit state, but there's not point because you're not changing anything, and in any case, it will drop out of Edit state when you do a .Next}

    dmregisteredusers.tblusers.First;
    while not dmregisteredusers.tblusers.Eof do
    {For k:= 1 to tblusers.RecordCount do <- Never, ever, iterate a dataset with a For loop.
     The RecordCount is best avoided, too, because not all types of dataset return a meaningful
     RecordCount }

    begin
      //  You need to decide what to do about users whose 'Dogs wanted' field is Null
      //  If is is Null maybe we should ask for the record to be changed to indicate
      // explicitly whether a dog is wanted or not

      if AField.IsNull then begin
        // to be filled in by you
      end;
      //  You haven't told us what DataType the 'Dogs wanted' field is
      //  There are several possibilities.  For simplicity, let's assume that the DataType of the field is ftString
      //  Let's also make a design decision that *only* if the field only contains a 'Y' in upper or lower
      //  case, then that means a dog is wanted.
      //  First copy the field's string value into a local variable so we don't have to keep getting it for the
      //  following operation;
      S := dmregisteredusers.tblUsers.FieldByName(scDogsWanted).AsString;
      S := Trim(S);  { Trim() is a statndard function which removes leading and trailing whitespace }
      Wanted := CompareText(S, 'Y') = 0; { CompareText does a case-insensitive comparison and returns zero if equal}
      If
        { (tblusers['Dogs wanted'] = '') OR (tblusers['Dogs wanted'] = ' ')
          OR (tblusers['Dogswanted'] = 0)}
        Wanted
      then
        {tblusers.Next <- this is in the wrong place, you want to do a .Next regardless of the outcome of the "if"}
      else
      begin
        inc(DogsWanted);
        {tblusers.Next;}
      end;//else
      dmregisteredusers.tblusers.Next;
    {end;//with}
  end;//for
  ShowMessage('There are ' + IntToStr(DogsWanted) + ' new dogs added to wishlist ,please contact the users regarding this matter and them remove the dogs from their wishlist !')
end;

Answer 2

我很欣赏你在答案中付出的努力我不能使用它们，因为我的导师会知道它不是我自己的工作...... @MartynA你的编码得到了我的问题的答案这是新的编码我知道它的草率和其他许多方法可以工作，但我真的很紧张，所以不能花时间学习代码......这是我将使用的

For k:= 1 to tblusers.RecordCount do
begin
If dmregisteredusers.tblusers.FieldByName('Dogs wanted').IsNull then tblusers.Next
else
    begin
   inc(i) ;
   tblusers.Next;
   end//then begin

我使用db form.onshow的排序，所以当DBGrid显示时它将按正确顺序...

什么数据类型是空数据库字段？

2 个答案: