Get the result from the Query - delphi

Question

I write this code :
Var Q : TFDQuery;
begin
Q := TFDQuery.Create(Self);
Q.Connection := FDConnection1;
Q.Params.CreateParam(ftString,'N',ptOutput);// Try also ptResult
Q.Params.CreateParam(ftInteger,'ID',ptInput);
Q.SQL.Text := 'SELECT NOM FROM EMPLOYEE WHERE ID_EMP = :ID';
Q.Params.ParamByName('ID').Value := 1;
Q.Active := True;
ShowMessage( VarToStr(Q.Params.ParamByName('N').Value) );
The result should be the name of the employer.
I get an error :
'N' parameter not found
How can I get the result from the Query using the parameter?
If I can't , what is the the function of :
ptOutput
ptResult

Answer 1

Try this code:
procedure TForm1.ExecuteQuery;
var
SQL : String;
Q : TFDQuery;
begin
SQL := 'select ''Sami'' as NOM'; // Tested with MS Sql Server backend
try
Q := TFDQuery.Create(Self);
Q.Connection := FDConnection1;
Q.Params.CreateParam(ftString, 'Nom', ptOutput);// Try also ptResult
Q.SQL.Text := SQL;
Q.Open;
ShowMessage( IntToStr(Q.ParamCount));
Caption := Q.FieldByName('Nom').AsString;
finally
Q.Free; // otherwise you have a memory leak
end;
end;
You'll see that the created parameter no longer exists once the FDQuery is opened, because FireDAC "knows" that there is nothing it can do with it.
Then, replace Q.Open by Q.ExecSQL. When that executes you get an exception
with the message
Cannot execute command returning result set.
Hint: Use Open method for SELECT-like commands.
And that's your problem. If you use a SELECT statement, you get a result set whether
you like it or not, and the way to access its contents is to do something like
Nom := Q.FieldByName('Nom').AsString
You asked in a comment what is the point of ptOutput parameters. Suppose your database has a stored procedure defined like this
Create Procedure spReturnValue(#Value varchar(80) out)
as
select #Value = 'something'
Then, in your code you could do
SQL := 'exec spReturnValue :Value'; // note the absence of the `out` qualifier in the invocation of the SP
try
Q := TFDQuery.Create(Self);
Q.Connection := FDConnection1;
Q.Params.CreateParam(ftString, 'Value', ptOutput);// Try also ptResult
Q.SQL.Text := SQL;
Q.ExecSQL;
ShowMessage( IntToStr(Q.ParamCount));
Caption := Q.ParamByName('Value').AsString;
finally
Q.Free; // otherwise you have a memory leak
end;
which retrieves the output parameter of the Stored Proc into Q's Value parameter.

Answer 2

There is no need to manually create parameters. Data access components are smart enough to parse the SQL string and populate the parameters collection by themselves
Also to get the result you must read the query's fields. When you call Open on a Query component, the fields collection will be populated with the fields that you specified in the SELECT [fields] SQL statement
As a side note, I advice that you use the type-safe version to get the value from a TField or TParameter object: See more here
var
q : TFDQuery;
begin
q := TFDQuery.Create(Self);
q.Connection := FDConnection1;
q.SQL.Text := 'SELECT NOM FROM EMPLOYEE WHERE ID_EMP = :ID';
q.ParamByName('ID').AsInteger := 1;
q.Open;
ShowMessage(q.FieldByName('Nom').AsString);
end;