Issue With Subquery Inside Target Table Expression Column

As stated, I am getting warnings which turn into run failures due to subquery(s) inside expression column of target table. The decode query looks something like:

DECODE((SELECT alias.column FROM actualTable alias WHERE alias.differentColumn = actualTable.column),........

The warning I get from this looks like:

Unresolved Identifiers: alias.column, alias.differentColumn

This issue doesn't exist in the same mapping I have in 11g and my thoughts were to use a subquery filter but I'm not entirely sure how I would go about doing that. Any insight would be greatly appreciated.