The Rules According to ANSI
The ANSI standard declares that views are read-only (not modifiable) if the CREATE VIEW statement contains any of the following:
DISTINCT in the select list
Expressions (computed columns, aggregates, functions, and so on) in the select list
References to more than one table—in the FROM clause or in a subquery or in a UNION clause
References to a view that is itself not updatable, either in the FROM clause or in a subquery
A GROUP BY or HAVING clause
Some dialects of SQL are less restrictive than the ANSI standard; others may be more restrictive. You can consult your reference manual, or you can experiment. When you try to modify data through a view, SQL checks to make sure that no restrictions are violated. If it detects ...