You can find a patch for the bro "CURRENT" release which enables you to log in a MySQL database at http://manux.rstack.org/bro_mysql.
It requires the library MySQL++.
It uses a new bro bif called log_external which logs alerts according to a method passed as an argument.
With this patch the only supported method is ALERT_LOG_EXTERNAL_SQL which logs in a MySQL database.
A basic bro script that uses this method is given as an example (policy/test.bro).
For instance:
[paul@duncan bro-pub-0.8a48.dev-mysql]$ ./bro -r example-attacks/ftp-site-exec.trace test
976284129.459304 0.91239 other-3914 431 0 38.33.11.127 131.243.169.116 SF X
976284140.775142 0.567636 other-3915 304 0 38.33.11.127 131.243.169.116 SF X
...