[Miguel]

pawn Код:

new playername[MAX_PLAYER_NAME], query[128];
GetPlayerName(playerid, playername, sizeof(playername)); 
mysql_real_escape(playername, playername);
format(query, sizeof(query), "SELECT * FROM `usuarios` WHERE UserName = '%s'", playername); 
mysql_query(query);
mysql_store_result();
if(mysql_num_rows() == 1) PlayerInfo[playerid][Registered] = 1;
else PlayerInfo[playerid][Registered] = 0;
mysql_free_result();

That's how you should do it.