Updatable Views

MIMIER's Updatable Views

转自：http://developer.mimer.com/documentation/html_92/Mimer_SQL_Engine_DocSet/Data_manipulation6.html

INSERT, UPDATE and DELETE statements may be used on views.

The operation is then performed on the base table upon which the view is defined. However, certain views may not be updated (for example a view containing DISTINCT values, where a single row in the view may represent several rows in the base table).

A view is not updatable if any of the following conditions are true:

• the keyword DISTINCT is used in the view definition
• the select list contains components other than column specifications, or contains more than one specification of the same column
• the FROM clause specifies more than one table reference or refers to a non-updatable view
• the GROUP BY clause is used in the view definition
• the HAVING clause is used in the view definition

Note: By defining an INSTEAD OF trigger any view can be made updatable. If all the INSTEAD OF triggers on the view are dropped, the view will revert to not updatable if one or more of the conditions are true.

SQL Standards Updatable Views

In order to be updatable according to the SQL standard, a view must:

• Have only one table T in its top-level FROM clause
• Not use SELECT DISTINCT in its top-level FROM clause
• Include all attributes from T that do not permit NULLs
• Not refer to T in subqueries
• Not use GROUP BY or aggregation