Update with aggregate subquery?

Hello all -

I'm trying to find a way to simply some code, and I have an idea that I
can't quite hatch. It might be too complex for SQL.

I'm trying to do something like

UPDATE first_table SET latitude = ( SELECT avg(lat) FROM another_table
GROUP BY another_table.first_table_id WHERE another_table.first_table_id
= first_table.id )

Basically I have to refer to the 'outside' table within the subselect --
the 'first_table.id' in the subselect. Is this possible?

The added wrinkle is that the table I am selecting from is the same
table in a self join! Would that add any problems?