ZeosLib - handling exceptions for TConnection and TZQuery
lazarus / 16-Databases

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M[a]nny · 2011-12-15 09:17

Hello!

I use Zeslib for MySQL database connection. I do it with this code (C
is TConnection component already placed in main Form):

Code: [Select]var
  Q: TZQuery;
begin
  { If we are online }
  if not C.Connected then begin
        C.HostName := '127.0.0.1';
    C.User := 'root';
    C.Password := '';
    C.Database := 'mydb';
    C.Catalog := 'mydb';

    try
        C.Connect;
    except
      on E: Exception do begin // catching exeptions
        C.Disconnect; // an error has happened => we are going to
offline status
        if (E.ClassName = 'EZSQLException') and (Copy(E.Message, 0,
33) = 'SQL Error: Access denied for user') then
                ShowError('Access denied. You have propably entered
incorrect database connection information.', 'Connection error')
                else
            if (E.ClassName = 'EZSQLException') and
(GetMySQLErrorId(E.Message) = '10061') then
              ShowError('Connection to database cannot be done.',
'Connection error')
        else
          ShowError('<unhandled connection message> '+E.Message);
        end;
    end;
  end;

  { If we are already online }
  if C.Connected then begin
    Q := TZQuery.Create(nil);
    Q.Connection := C;

    Q.SQL.Text := 'SELECT my_column FROM my_table WHERE id=50;';
    try
      Q.Open;
      ShowInfo(Q.FieldByName('my_column').AsString);
      Q.Close;
    except
      on E: Exception do begin // catching exeptions
        C.Disconnect; // an error has happened => we are going to
offline status
        if E.Message = 'SQL Error: MySQL server has gone away' then
          ShowError('Connection to database has been lost.',
'Connection error')
        else
                ShowError('<unhandled query message> '+E.Message);
        end;
    end;

    Q.Free;
  end;
This code works OK but if I want make query, i have to write all these
lines. Is there any other way? For example i would write only:

Code: [Select]var
  Q: TZQuery;
begin
  Q := TZQuery.Create(nil);
  Q.Connection := C;
 
  Q.SQL.Text := 'SELECT my_column FROM my_table WHERE id=50;';
  Q.Open;
  ShowMessage(Q.FieldByName('my_column').AsString);
  Q.Close;
  Q.Free;
... and somewhere else in the code (in global placement which is
called everytime when query is trying to make database transaction)
will be done code above - for testing wether Connection is online and
handling exceptions for connection and for query aswell.

I hope i wrote it clearly.