NullColumn

Derives a new column with a different owner with this column as underlying.

Used to represent columns on the outside of subqueries and common table expressions. Note that column aliases are tracked across derivation so that derived columns will continue to refer to the same underlying data irrespective of renaming.

Equality on SqlColumns

Two SqlColumns are equal if their underlyings have the same name and owner.

Yields a copy of this column in other

Only well defined if the move doesn't lose an owner name

Is this column a reference to a column of a table

The named owner of this column, if any

This column as a Term which can appear in a Predicate