group by & order by rand() problem

[[EMAIL PROTECTED]]

I have this table:

mysql> select * from banners;
+----+---------+---------------+
| id | user_id | title         |
+----+---------+---------------+
|  1 |       1 | first banner  |
|  2 |       1 | second banner |
|  3 |       2 | third banner  |
|  4 |       2 | forth banner  |
|  5 |       2 | fifth banner  |
+----+---------+---------------+

I would like to show a random banner for each user,
something like this:

first call
+----+---------+---------------+
| id | user_id | title         |
+----+---------+---------------+
|  1 |       1 | first banner  |
|  3 |       2 | third banner  |
+----+---------+---------------+

second call
+----+---------+---------------+
| id | user_id | title         |
+----+---------+---------------+
|  2 |       1 | second banner |
|  4 |       2 | forth banner  |
+----+---------+---------------+

etc...

I have tried with following query but the banner
doesn't change while multiple calls:

SELECT * FROM banners GROUP BY user_id ORDER BY RAND();

Can anyone help me?

Thanks in advance,

Alex

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