Must declare scalar variable @tablename when updating another table

Question

I am trying to do:

update existingTable
set code = @declaredTable.code
where id = @declaredTable.id

but when I do this I get the error:

Must declare the scalar variable "@declaredTable"

twice.

How do I update values of an existing tables to values from a declared tables?

Why doesn't this work?

Any help would be much appreciated!

Answer 1

You need JOIN :

UPDATE e 
      SET e.code = d.code
FROM existingTable e INNER JOIN 
     @declaredTable d
     on d.id = e.id;