public Cursor getAllDataTableStaffDatabase(String table_name){
     this.open();
     Cursor result =this.mDb.rawQuery("SELECT * FROM " + table_name,null);
     return result;// fonctionne très bien
}

public String findNameOfStaffBymail(String mail) {
    String sql = " SELECT  *  FROM " + DatabaseStaffHandler.STAFF_TABLE_NAME + " WHERE " + DatabaseStaffHandler.STAFF_MAIL + " = ? ";
    Cursor result = super.mDb.rawQuery(sql, new String[]{mail});
    Cursor data = super.getAllDataTableStaffDatabase(DatabaseStaffHandler.STAFF_TABLE_NAME);

    String test = result.getString(1); //error

    while (data.moveToNext()) {
        if (data.getString(3).equals(mail)) {
            viewAll();
            return data.getString(1);
        }
    }
}