How to get a list of unique values for a given column from TDataset
lazarus / 16-Databases

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stoffman · 2021-11-04 14:11

Hi,

Is there a "standard" way to get all the unique values for a given
column in TDataset? I can loop my self and find them, but I'm sure I'm
not the first one to encounter this problem so I'm looking for more
official way.

EDIT: To be sure the data source is not a database, and I explicitly
wish to achieve this with data already loaded into a dataset  

Thanks,

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SymbolicFrank · 2021-11-04 14:31

I would add them to a TStringList with duplicates turned off.

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devEric69 · 2021-11-04 14:41

Depending on the database, the SQL keyword DISTINCT or DISCTINCTROW
exists.

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Zvoni · 2021-11-04 15:03

Nevermind, if that column is already defined as "UNIQUE" then you just
SELECT it, since the DB enforces uniqueness of values for that column

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stoffman · 2021-11-04 15:47

@zvoni @devEric69

The data source is not a database.

@SymbolicFrank

That seems like an option. Are there any alternative (assuming the
column is not a string and I would like to avoid the conversion) ?

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mangakissa · 2021-11-05 14:46

Quote from: stoffman on November 04, 2021, 03:11:08 pmHi,

Is there a "standard" way to get all the unique values for a given
column in TDataset? I can loop my self and find them, but I'm sure I'm
not the first one to encounter this problem so I'm looking for more
official way.

EDIT: To be sure the data source is not a database, and I explicitly
wish to achieve this with data already loaded into a dataset  

Thanks,
Do you fill the TDataset with a file?
If it is you have to manage it before loading it. If the dataset is
already loaded and want to get unique values, SymbolicFrank gives you
the only way. Must the data showed in the grid after finding
uniqueness?

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stoffman · 2021-11-05 15:37

Quote from: mangakissa on November 05, 2021, 03:46:18 pmDo you fill
the TDataset with a file?
If it is you have to manage it before loading it. If the dataset is
already loaded and want to get unique values, SymbolicFrank gives you
the only way. Must the data showed in the grid after finding
uniqueness?

Yes, from a file. And as I said I explicitly wish to achieve this
after the data already loaded into the dataset.

The dataset may or may not be shown in a Grid. Why does it affect the
solution?

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avk · 2021-11-05 15:46

Didn't check if this would work:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  uses  ...,
Generics.Collections;...type  TVarList = specialize
TList<Variant>; function GetUniqList(d: TDataSet; const aFieldName:
string): TVarList;var  s: specialize THashSet<Variant>;  v:
Variant;begin  Result := TVarList.Create;  s := specialize
THashSet<Variant>.Create;  try    d.First;    while not d.EOF do     
begin        v := d.FieldByName(aFieldName).Value;        if s.Add(v)
then          Result.Add(v);        d.Next;      end;  finally   
s.Free;  end;end;