Receiving data from an SQL table in Delphi - mysql

Question

I have a form which needs to fetch and display data from a MYSQL database.
The SQL code which I have written works fine, as I have tried to execute it within MYSQL.
I have also used a similar format of code to the one in my form, within another form, to fetch and display SQL data. The code works perfectly in the other form.
However, my code for this form does not seem to be receiving/displaying the data from the SQL table.
Here's a snippet of the code written for the OnShow event of the form.
procedure TfrmEditBooking.FormShow(Sender: TObject);
var
CustName: string;
begin
if SelectedID > 0 then
begin
with frmBookingData.ADOQuery1 do
begin
Open;
SQL.Clear;
SQL.Add('SELECT Name, Surname ' +
'FROM customers_main ' +
'WHERE customers_main.idcustomers_main ' +
'IN ' +
'(SELECT bookings_main.customers_main_idcustomers_main ' +
'FROM bookings_main ' +
'WHERE bookings_main.idbookings_main = ' + IntToStr(SelectedID) + ')');
ExecSQL;
CustName := FieldByName('Surname').AsString + ' ' + FieldByName('Name').AsString;
Label1.Caption := CustName;
Close;
end;
end;
end;

Answer 1

One uses 'execsql' when one is not expecting to receive a cursor into a dataset (in other words, use 'execsql' with 'update', 'insert' and 'delete' statements but not with 'select').
You should replace the 'execsql' command with 'open' ... and remove the 'open' before 'sql.clear'.
Don't use a subquery when you should be using a join.
I think that your code ought to look like this
procedure TfrmEditBooking.FormShow(Sender: TObject);
begin
if SelectedID > 0 then
with frmBookingData.ADOQuery1 do
begin
SQL.Clear;
SQL.Add ('SELECT Name, Surname ');
sql.add ('FROM customers_main inner join bookings_main');
sql.add ('on customers_main.idcustomers_main = ');
sql.add ('bookings_main.customers_main_idcustomers_main');
sql.add ('where bookings_main.idbookings_main = :p1');
sql.params[0].asinteger:= SelectedID;
open;
Label1.Caption := fieldbyname ('name').asstring + ' ' +
fieldbyname ('surname').asstring;
Close;
end;
end;

Answer 2

To get this running you should change it to
procedure TfrmEditBooking.FormShow(Sender: TObject);
var
CustName: string;
begin
if SelectedID > 0 then
begin
with frmBookingData.ADOQuery1 do
begin
Close; // close first
SQL.Clear;
SQL.Add(
'SELECT Name, Surname '+
'FROM customers_main '+
'WHERE customers_main.idcustomers_main '+
'IN '+
'(SELECT bookings_main.customers_main_idcustomers_main '+
'FROM bookings_main '+
'WHERE bookings_main.idbookings_main = '+IntToStr(SelectedID)+')');
Open; // open the query
if not Eof then
CustName := FieldByName('Surname').AsString+' '+FieldByName('Name').AsString
else
CustName := 'not found';
Close; // close when finished
end;
Label1.Caption := CustName;
end;
end;
But you should get some (negative) side effects, if frmBookingData.ADOQuery1 is already in use for something different