How to get a value using SQL in Delphi and setting the value to a variable?

问题

I'm trying to get the price of medication from the table but i just get:

procedure TForm1.BuyButtonClick(Sender: TObject);
var
  iAmount : integer;
  rRate : real;
  sMedication : string;
  sRate : string;
begin
  iAmount := 0;
  sMedication := BuyCombobox.Items[BuyCombobox.ItemIndex];
  dmHospital.qryPrices.SQL.Clear;
  dmHospital.qryPrices.SQL.Add('SELECT Price(R) FROM MedicationPrices WHERE Medication = quaotedstr(sMedication)');
  sRate := dmHospital.qryPrices.SQL;
  ShowMessage(sRate);
end;

回答1:

You're not using the query properly. qryPrices.SQL is the SQL statement itself. It's just text. You need to do something to actually run the statement. (See below.)

You've also embedded the variable inside the quotes, which means it's not being evaluated, and neither is the function call to the (misspelled) QuotedStr. There is no function quaotedStr(). If you insist on the poor idea of concatenating SQL, you need to do it properly. If you're going to clear and then add, you can just assign to SQL.Text instead to do it in one step:

dmHospital.qryPrices.SQL.Text := 'SELECT Price(R) FROM MedicationPrices WHERE Medication = ' + Quotedstr(sMedication);

Also, the query won't do anything until you actually execute it. You need to use qryPrices.Open to run a SELECT statement, or qryPrices.ExecSQL to run an INSERT, UPDATE or DELETE statement.

You should get out of the thought of concatenating SQL immediately (before you get the habit) and learn to use parameterized queries. It allows the database driver to handle the formatting and conversion and quoting for you, and it also prevents SQL injection that can give others access to your data. Here's a corrected version that should get you started.

procedure TForm1.BuyButtonClick(Sender: TObject);
var
  sMedication : string;
  sRate : string;
begin
  iAmount := 0;
  sMedication := BuyCombobox.Items[BuyCombobox.ItemIndex];
  dmHospital.qryPrices.SQL.Text := 'SELECT Price(R) FROM MedicationPrices WHERE Medication = :Medication';
  dmHospital.qryPrices.Parameters.ParamByName('Medication').Value := sMedication;
  dmHospital.qryPrices.Open;
  sRate := dmHospital.qryPrices.FieldByName('Price(R)').AsString;
  dmHospital.qryPrices.Close;
  ShowMessage(sRate);
end;

回答2:

You should modify Your code to actually work: My advise is to use parameters instead of QuotedStr:

dmHospital.qryPrices.SQL.Clear;
dmHospital.qryPrices.SQL.Add('SELECT Price(R) AS Rate FROM MedicationPrices WHERE   Medication = :pMedication');
dmHospital.qryPrices.Params.ParamByName('pMedication').AsString=sMedication;

(Note that in ADOQuery You'd use .Parameters instead of .Params)

dmHospital.qryPrices.Open;
sRate=dmHospital.qryPrices.FieldByName('Rate').AsString;
ShowMessage(sRate);

Regards

回答3:

Not tested it (dont have Delphi at hand here) but it should be something like this :

iAmount := 0;  
sMedication := BuyCombobox.Items[BuyCombobox.ItemIndex];  
dmHospital.qryPrices.SQL.Clear;  
dmHospital.qryPrices.SQL.Add('SELECT Price(R) as price FROM MedicationPrices WHERE Medication = ' + QuotedStr(sMedication));  

dmHospital.qryPrices.Open;
if (dmHospital.qryPrices.RecordCount = 1)
  sRate := dmHospital.qryPrices.FieldByName('price').AsString;  

ShowMessage(sRate);  

来源：https://stackoverflow.com/questions/38145959/how-to-get-a-value-using-sql-in-delphi-and-setting-the-value-to-a-variable