What restrictions are necessary to ensure that a view is updatable?

What will be an ideal response?

ISO standard specifies the views that must be updatable in a system that conforms to the standard.
Definition given in SQL standard is that a view is updatable if and only if:
? DISTINCT is not specified; that is, duplicate rows must not be eliminated from the query
results.
? Every element in the SELECT list of the defining query is a column name (rather than a
constant, expression, or aggregate function) and no column appears more than once.
? The FROM clause specifies only one table; that is, the view must have a single source table
for which the user has the required privileges. If the source table is itself a view, then that
view must satisfy these conditions. This, therefore, excludes any views based on a join, union
(UNION), intersection (INTERSECT), or difference (EXCEPT).
? The WHERE clause does not include any nested SELECTs that reference the table in the
FROM clause.
? There is no GROUP BY or HAVING clause in the defining query.