Reference column from another table for a List

Dear All,

I have the following table:

Table1

What I want is:

in column CODE, replace "null" with a list of all the distinct codes existent in column CODE except "null".

MSo instead of nulls I get a list of the following elements {724, 323, 455, 444, 256} 

I achieve this throught the following step:

= Table.TransformColumns( #"previous step", {{"CODE", (x)=> if x = null then List.Select(List.Distinct( #"previous step"[CODE]), each _<> null) else {x}}})

Now, I have another table with a list of codes:

Table2

My question is: How do I reference CODE column in Table 2 in the previous step INSTEAD of CODE column in Table1?

Thanks for your time

gianmarco