SQLiteHelper sqlite = new SQLiteHelper();

if (!sqlite.CheckAdmin())
{
[...]
}
User login = sqlite.Login(Loginname, HelperClass.MD5Hash(Password));

class SQLiteHelper
    {
        private SQLiteConnection sqlConn;
        private static string sqliteFile = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData) + "/database.db";  // set folder for database
        private static string sqlitePw = "databasepw"; // set password for database

        public SQLiteHelper()
        {
            // check if database file exist when not create with password
            if (!File.Exists(sqliteFile))
            {
                sqlConn = new SQLiteConnection("Data Source=" + sqliteFile);
                sqlConn.SetPassword(sqlitePw);
            }
            sqlConn = new SQLiteConnection("Data Source=" + sqliteFile + ";Password=" + sqlitePw); // connect to database

            // create tables, when not exist
            string query =
                "CREATE TABLE IF NOT EXISTS user (id INTEGER PRIMARY KEY AUTOINCREMENT, 'login' TEXT, 'firstname' TEXT, 'lastname' TEXT, 'persid' TEXT, 'password' Text, 'role' INTEGER, 'first' INTEGER, 'active' INTEGER);" +
                "CREATE TABLE IF NOT EXISTS userRigths (id INTEGER PRIMARY KEY, 'lwe' INTEGER, 'lwa' INTEGER, 'lwb' INTEGER, 'lwi' INTEGER, 'mwe' INTEGER, 'mwa' INTEGER, 'mwb' INTEGER, 'mwi' INTEGER, 'vacc' INTEGER, 'vadr' INTEGER);" +
                "CREATE TABLE IF NOT EXISTS admin ('password' Text);";
            queryNon(query);
        }


        // check if admin password exist
        public bool CheckAdmin()
        {
            sqlConn.Open();
            var command = sqlConn.CreateCommand();
            command.CommandText = "SELECT * FROM admin";
            SQLiteDataReader reader = command.ExecuteReader();
            bool rows = reader.HasRows;
            sqlConn.Close();
            return rows;
        }

        // login user
        // if login doesnt match return an empty user
        public User Login(string login, string pw)
        {
            User user = new User();

            sqlConn.Open();

            if (!login.Equals("Admin"))
            {
                var command = sqlConn.CreateCommand();
                command.CommandText = "SELECT * FROM user LEFT JOIN userRigths ON user.id = userRigths.id WHERE user.login = '" + login + "' AND user.password = '" + pw + "'";
                SQLiteDataReader reader = command.ExecuteReader();
                if (reader.HasRows)
                {
                    while (reader.Read())
                    {
                        user.Id = reader.GetInt32(0);
                        user.Login = reader.GetString(1);
                        user.Role = reader.GetInt16(6);
                        break;
                    }
                }
            }
            else
            {
                var command = sqlConn.CreateCommand();
                command.CommandText = "SELECT * FROM admin WHERE admin.password = '" + pw + "'";
                SQLiteDataReader reader = command.ExecuteReader();
                if (reader.HasRows)
                {
                    user.Id = 0;
                    user.Login = "Admin";
                    user.Role = -1;
                }
            }

            sqlConn.Close();

            return user;
        }

        // methode for querys without response
        private void queryNon(string query)
        {
            sqlConn.Open();
            var command = sqlConn.CreateCommand();
            command.CommandText = query;
            command.ExecuteNonQuery();
            sqlConn.Close();
        }
    }

command.CommandText = "SELECT * FROM user LEFT JOIN userRigths ON user.id = userRigths.id WHERE user.login = '"
    + login + "' AND user.password = '" + pw + "'";

command.CommandText = "SELECT * FROM user LEFT JOIN userRigths ON user.id = userRigths.id"
                    + "WHERE user.login = @login AND user.password = @password";
command.Parameters.AddWithValue("@login", login);
command.Parameters.AddWithValue("@password", pw);
SQLiteDataReader reader = command.ExecuteReader();

ApplicationDatabase database = new ApplicationDatabase();

if (!database.CheckAdmin())
{
[...]
}
User login = database.Login(Loginname, HelperClass.MD5Hash(Password));

public ApplicationDatabase()
{
    if (!DatabaseExists())
    {
        CreateDatabase(databaseFile, databasePassword);
        CreateTables();
    }
    ConnectToDatabase();
}

public User Login(string login, string password)
{
    if (!login.Equals("Admin"))
    {
        user = GetUser(login, password);
    }
    else
    {
        user = GetAdmin(login, password);
    }
    return user;
}

public bool AdminPasswordExists(){}
private void QueryWithoutResponse(string query){}