foreign key check errors

[smgogarten]

If a foreign key refers to a column that can have duplicate values, we get a validation error because that column is expected to be a unique key. Use same workaround as for set tables, by checking if column is expected to be unique?

Also if a column that references a foreign key has all missing values, we get the following error:
Can't join x$value1 with y$value1 due to incompatible types.
x$value1 is a .
y$value1 is a .