Manage views

This topic describes how to create, modify, and drop views. A view displays the result of a query. You can use views in most cases where you can use tables. If the data you frequently access is distributed across multiple tables, a view is the best choice.

Create a normal view

You can execute the CREATE VIEW statement to create a normal view. After a view is created, you can execute DML statements on it.

Syntax for creating a view in MySQL mode:

create_view_stmt:
  CREATE [OR REPLACE] VIEW view_name [(column_name_list)] AS select_stmt;
  [WITH [CASCADED | LOCAL] CHECK OPTION];
column_name_list:
    column_name [, column_name ...]

Parameters:

• OR REPLACE: If the name of the view to be created already exists, a new view is created based on the specified definition.

• select_stmt: defines the SELECT statement of the view. You can obtain the information to be used in this statement from a base table or other views.

• column_name_list: the unique column names used in the view. Each base table must also have unique column names. By default, the column names retrieved by the SELECT statement are used as the column names in the view.

  You can also use the optional column_name_list clause to define column names for a view. Separate multiple column names with commas (,). The number of column names in column_name_list must be equal to the number of columns retrieved by the SELECT statement.

  The SELECT statement can directly reference the columns of a table. You can also use functions, constants, and operators in the statement to retrieve columns.

• The WITH CHECK OPTION clause prevents the system from inserting or updating rows except for the rows that meet the query condition defined by the WHERE clause in select_statement. When you create a view by using the WITH CHECK OPTION clause, you can add the CASCADED or LOCAL keyword to clarify the check scope.

  • If the LOCAL keyword is used, the WHERE clause of a view is checked, then checking recurses to underlying views and applies the same rules.

  • If the CASCADED keyword is used, the WHERE clause of a view is checked, then checking recurses to underlying views, adds WITH CASCADED CHECK OPTION to these views without changing their definitions, and applies the same rules.

  • If neither the CASCADED nor the LOCAL keyword is used in WITH CHECK OPTION, the CASCADED keyword takes effect by default.

  • If the WITH CHECK OPTION clause is not used when you create a view, the WHERE clause of this view is not checked, then checking recurses to underlying views and applies the same rules.

Example:

obclient> CREATE VIEW stock_item
     AS
     SELECT /*+ leading(s) use_merge(i) */
     i_price, i_name, i_data, s_i_id, s_w_id, s_order_cnt, s_ytd, s_remote_cnt, s_quantity, s_data, s_dist_01, s_dist_02, s_dist_03, s_dist_04, s_dist_05, s_dist_06, s_dist_07, s_dist_08, s_dist_09, s_dist_10
     FROM stok s, item i
     WHERE s.s_i_id = i.i_id;
Query OK, 0 rows affected

Create a read-only view

You can add the WITH READ ONLY keyword to the CREATE OR REPLACE VIEW statement to create a read-only view. You cannot execute any DML statements on a read-only view, including INSERT, UPDATE, DELETE, and REPLACE.

Example: Create a read-only view named view1.

obclient>  CREATE OR REPLACE VIEW view1 AS SELECT * FROM tab1 WITH READ ONLY;

Modify a normal view

You can execute the CREATE OR REPLACE VIEW statement to modify a normal view.

Example: Modify the stock_item view.

obclient> CREATE OR REPLACE VIEW stock_item
     AS
     SELECT /*+ leading(s) use_merge(i) */
     i_price, i_name, i_data, s_i_id, s_w_id, s_order_cnt, s_ytd, s_remote_cnt, s_quantity, s_data, s_dist_01, s_dist_02, s_dist_03, s_dist_04, s_dist_05, s_dist_06, s_dist_07, s_dist_08, s_dist_09, s_dist_10
     FROM stok s, item i
     WHERE s.s_i_id = i.i_id;
Query OK, 0 rows affected (0.02 sec)

Drop a view

You can execute the DROP VIEW statement to drop one or more views at a time. When a view is dropped, tables referenced by the view are not dropped.

If you drop a view that is referenced by another view, queries on the view that references the dropped view will fail.

Before you drop a view, make sure that you have the DROP privilege on the view.

Syntax for dropping a view in MySQL mode:

drop_view_stmt:
    DROP VIEW [IF EXISTS] view_name_list;

view_name_list:
    view_name [, view_name_list]

Note:

• The IF EXISTS keyword prevents the error that occurs because the view to be dropped does not exist.

• If view_name_list contains views that do not exist, an error may be reported during execution but the specified views that exist are dropped.

Example: Drop the V1 view.

obclient> DROP VIEW V1;