Mysql variable in subselect problem

Mysql subquery variable missing.sql

SELECT @a := test,
   (SELECT @a) AS first_level_subquery,
   (SELECT * FROM (SELECT @a) as tmp ) AS second_level_subquery
FROM (
    SELECT 1 as test UNION ALL SELECT 2 UNION ALL SELECT 3
) AS some_data\G

+------------+----------------------+-----------------------+
| @a := test | first_level_subquery | second_level_subquery |
+------------+----------------------+-----------------------+
|          1 | 1                    | NULL                  |
|          2 | 2                    | NULL                  |
|          3 | 3                    | NULL                  |
+------------+----------------------+-----------------------+