Apt dependency hell

[Roger H. Goun]

I have a Ubuntu dapper server that runs a lot of Web sites. It hasn't 
had it's mysql packages updated since early in dapper development, when 
mysql4.1 was the default version. Trying to apt-get upgrade to the 
mysql5 versions broke the system and I can't figure out how to get back 
to a working state.

For example:

roger at river:/var/lib/dpkg$ sudo apt-get update
...
roger at river:/var/lib/dpkg$ sudo apt-get install sql-common
Reading package lists... Done
Building dependency tree... Done
Note, selecting mysql-common-4.1 instead of mysql-common
Package mysql-common-4.1 is a virtual package provided by:
You should explicitly select one to install.
E: Package mysql-common-4.1 has no installation candidate

Does anyone know how to clean up apt's notion of what virtual packages 
are available?

Thanks.

-- Roger