mysql> SELECT
    ->   LEFT(rc_timestamp, 8) AS day,
    ->   COUNT(rc_timestamp) AS users
    -> FROM recentchanges
    -> WHERE rc_log_type = 'newusers'
    -> GROUP BY day;
+----------+-------+
| day      | users |
+----------+-------+
| 20110921 |    17 |
| 20110922 |    46 |
| 20110923 |    29 |
| 20110924 |    21 |
| 20110925 |    24 |
| 20110926 |    36 |
| 20110927 |    50 |
| 20110928 |    31 |
| 20110929 |    29 |
| 20110930 |    32 |
| 20111001 |    20 |
| 20111002 |    26 |
| 20111003 |    42 |
| 20111004 |    49 |
| 20111005 |    41 |
| 20111006 |    50 |
| 20111007 |    44 |
| 20111008 |    33 |
| 20111009 |    31 |
| 20111010 |    64 |
| 20111011 |    43 |
| 20111012 |    32 |
| 20111013 |    36 |
| 20111014 |    28 |
| 20111015 |    36 |
| 20111016 |    26 |
| 20111017 |    36 |
| 20111018 |    36 |
| 20111019 |    62 |
| 20111020 |    53 |
| 20111021 |   426 |
+----------+-------+
31 rows in set (0.03 sec)