[Jack_Leslie]

Here's mine that works as an example:

pawn Код:

forward IsAccountRegistered(playerid);
public IsAccountRegistered(playerid)
{
    new rows, fields, string[256];
    cache_get_data(rows, fields, Mysql_users);
    
    
    if(rows) // If account is registered
    {
        mysql_format(Mysql_users, query, sizeof(query), "SELECT * FROM `fortcarson` WHERE `Fullname` = '%e' LIMIT 1", GetPlayersName(playerid));
        mysql_tquery(Mysql_users, query, "Loadplayerdata", "i", playerid);
        format(string, sizeof(string), "{FFFFFF}Hello, %s!\n\nWelcome back to {D69929}Fort Carson{FFFFFF}.\nPlease login with your existing password below.", GetPlayersName(playerid));
        ShowPlayerDialog(playerid, DIALOG_LOGIN, DIALOG_STYLE_PASSWORD, "{FFFFFF}Welcome back to Fort Carson.", string, "Login", "Leave");
    }
    else if(!rows) {
        format(string, sizeof(string), "{FFFFFF}Hello, %s.\n\nWelcome to {D69929}Fort Carson Roleplay!{FFFFFF}\nYour account doesn't seem to exist.\nPlease input a desired password below.", GetPlayersName(playerid));
        ShowPlayerDialog(playerid, DIALOG_REGISTER, DIALOG_STYLE_INPUT, "{FFFFFF}Welcome to Fort Carson.", string, "Register", "Leave");
    }
    return 1;
}

So as AiRaLoKa said, use cache_get_data, then if(rows).