A simple expression can be a column, pseudocolumn, constant, sequence number, or null value.
In addition to user schemas,
schema can also be PUBLIC to specify the attribute of a synonym for a table or a view. You can use PUBLIC to specify a public synonym in only a DML statement. You cannot use PUBLIC in a DDL statement.
NVARCHAR2 are not valid pseudocolumn data types.
The following list provides some valid simple expressions:
'this is a text string'
N 'this is an NCHAR string'