A value-expression is an Expression that resolves to a single
value, with some limitations that are described here. See
The generation-clause may reference other non-generated columns in the table, but it must not reference any generated column. The generation-clause must not reference a column in another table.
The generation-clause may invoke user-coded functions, if the functions meet the following requirements:
The generation-clause must not include subqueries.
If the generated column is part of a foreign key that references another table, the referential action must not specify SET NULL or SET DEFAULT, and the update rule must not specify ON UPDATE CASCADE.