ASA SQL User's Guide
Joins: Retrieving Data from Several Tables
Inner and outer joins
Transact-SQL outer joins (*= or =*)
If you define a view with an outer join, and then query the view with a qualification on a column from the null-supplying table of the outer join, the results may not be what you expect. The query returns all rows from the null-supplying table. Rows that do not meet the qualification show a NULL value in the appropriate columns of those rows.
The following rules determine what types of updates you can make to columns through views that contain outer joins:
INSERT and DELETE statements are not allowed on outer join views.
UPDATE statements are allowed on outer join views. If the view is defined WITH CHECK option, the update fails if any of the affected columns appears in the WHERE clause in an expression that includes columns from more than one table.