20.5.3 Updatable and Insertable Views

Some views are updatable and references to them can be used to specify tables to be updated in data change statements. That is, you can use them in statements such as UPDATE, DELETE, or INSERT to update the contents of the underlying table.

For a view to be updatable, there must be a one-to-one relationship between the rows in the view and the rows in the underlying table. There are also certain other constructs that make a view nonupdatable. To be more specific, a view is not updatable if it contains any of the following:

Aggregate functions (SUM(), MIN(), MAX(), COUNT(), and so forth)
DISTINCT
GROUP BY
HAVING
UNION or UNION ALL
Subquery in the select list
Certain joins (see additional join discussion later in this section)
Reference to nonupdatable view in the FROM clause
Subquery in the WHERE clause that refers to a table in the FROM clause
Refers only to literal values (in this case, there is no underlying table to update)
ALGORITHM = TEMPTABLE (use of a temporary table always makes a view nonupdatable)
Multiple references to any column of a base table

It is sometimes possible for a multiple-table view to be updatable, assuming that it can be processed with the MERGE algorithm. For this to work, the view must use an inner join (not an outer join or a UNION). Also, only a single table in the view definition can be updated, so the SET clause must name only columns from one of the tables in the view. Views that use UNION ALL are not permitted even though they might be theoretically updatable.

With respect to insertability (being updatable with INSERT statements), an updatable view is insertable if it also satisfies these additional requirements for the view columns:

There must be no duplicate view column names.
The view must contain all columns in the base table that do not have a default value.
The view columns must be simple column references. They must not be expressions, such as these:
```
3.14159
col1 + 3
UPPER(col2)
col3 / col4
(subquery)
```

MySQL sets a flag, called the view updatability flag, at CREATE VIEW time. The flag is set to YES (true) if UPDATE and DELETE (and similar operations) are legal for the view. Otherwise, the flag is set to NO (false). The IS_UPDATABLE column in the INFORMATION_SCHEMA.VIEWS table displays the status of this flag. It means that the server always knows whether a view is updatable.

If a view is not updatable, statements such UPDATE, DELETE, and INSERT are illegal and are rejected. (Even if a view is updatable, it might not be possible to insert into it, as described elsewhere in this section.)

The updatability of views may be affected by the value of the updatable_views_with_limit system variable. See Section 5.1.7, “Server System Variables”.

Earlier discussion in this section pointed out that a view is not insertable if not all columns are simple column references (for example, if it contains columns that are expressions or composite expressions). Although such a view is not insertable, it can be updatable if you update only columns that are not expressions. Consider this view:

CREATE VIEW v AS SELECT col1, 1 AS col2 FROM t;

This view is not insertable because col2 is an expression. But it is updatable if the update does not try to update col2. This update is permissible:

UPDATE v SET col1 = 0;

This update is not permissible because it attempts to update an expression column:

UPDATE v SET col2 = 0;

For a multiple-table updatable view, INSERT can work if it inserts into a single table. DELETE is not supported.

INSERT DELAYED is not supported for views.

If a table contains an AUTO_INCREMENT column, inserting into an insertable view on the table that does not include the AUTO_INCREMENT column does not change the value of LAST_INSERT_ID(), because the side effects of inserting default values into columns not part of the view should not be visible.