Author Juarez Rudsatz
Some like this ?

UPDATE TableIWantUpdate
SET FieldIWant = ct.FieldIWant + at.AddedValue
FROM CopyofTableIWantUpdate ct , AnotherTable at
WHERE ct.KeyField = at.ForeignKey

I donĀ“t know how others dbms handle this, but copying syntax from another dbms will make user more comfortable when migrating to Firebird. Or not ?

Relational theory tells us that you should never update
more than one column in any table with values from another
table (transitive dependency, I think). However, the
difference between theory and fact shows up in compound
primary keys (probably also theoretically unsound) and
other design compromises.

In short, being able to update from a select would be
a good thing.

We have answers.

Auf Widersehen

Juarez Rudsatz