You are correct in stating that the documentation only says half of the story… Recently I was getting this error ( ORA-00998 ) when creating a temporary work table, and after some troubleshooting I realized that not only functions and expressions must have a column alias, but pseudocolumns also must have a column alias:

Example:

create table my_table as select a.rowid, a.dummy from dual a;
> Error at Command Line:1 Column:32
> SQL Error: ORA-00998: must name this expression with a column alias

create table my_table as select a.rowid as the_rowid, a.dummy from dual a;
> table MY_TABLE created.

In a fairly complex query it took me sometime to figure it out.