CREATE VIEW

Purpose

Use the CREATE VIEW statement to define a view, which is a logical table based on one or more tables or views. A view contains no data itself. The tables upon which a view is based are called base tables.

You can also create an object view or a relational view that supports LOBs, object types, REFs, nested table, or varray types on top of the existing view mechanism. An object view is a view of a user-defined type, where each row contains objects, each object with a unique object identifier.

You can also create XMLType views, which are similar to an object views but display data from XMLSchema-based tables of XMLType.

Prerequisites

To create a view in your own schema, you must have the CREATE VIEW system privilege. To create a view in another user's schema, you must have the CREATE ANY VIEW system privilege.

To create a subview, you must have the UNDER ANY VIEW system privilege or the UNDER object privilege on the superview.

The owner of the schema containing the view must have the privileges necessary to either select, insert, update, or delete rows from all the tables or views on which the view is based. The owner must be granted these privileges directly, rather than through a role.

To use the basic constructor method of an object type when creating an object view, one of the following must be true:

• The object type must belong to the same schema as the view to be created.

• You must have the EXECUTE ANY TYPE system privileges.

• You must have the EXECUTE object privilege on that object type.

  
  

  See Also: SELECT , INSERT , UPDATE , and DELETE for information on the privileges required by the owner of a view on the base tables or views of the view being created

  
  

Syntax

create_view::=

Description of the illustration create_view.gif

(inline_constraint::= and out_of_line_constraint::=, object_view_clause::=, XMLType_view_clause ::=, subquery::=—part of SELECT, subquery_restriction_clause::=)

object_view_clause::=

Description of the illustration object_view_clause.gif

(inline_constraint::= and out_of_line_constraint::=)

XMLType_view_clause ::=

Description of the illustration XMLType_view_clause.gif


XMLSchema_spec::=

Description of the illustration XMLSchema_spec.gif


subquery_restriction_clause::=

Description of the illustration subquery_restriction_clause.gif

Semantics

OR REPLACE

Specify OR REPLACE to re-create the view if it already exists. You can use this clause to change the definition of an existing view without dropping, re-creating, and regranting object privileges previously granted on it.

INSTEAD OF triggers defined in the view are dropped when a view is re-created.

If any materialized views are dependent on view, then those materialized views will be marked UNUSABLE and will require a full refresh to restore them to a usable state. Invalid materialized views cannot be used by query rewrite and cannot be refreshed until they are recompiled.

FORCE

Specify FORCE if you want to create the view regardless of whether the base tables of the view or the referenced object types exist or the owner of the schema containing the view has privileges on them. These conditions must be true before any SELECT, INSERT, UPDATE, or DELETE statements can be issued against the view.

If the view definition contains any constraints, CREATE VIEW ... FORCE will fail if the base table does not exist or the referenced object type does not exist. CREATE VIEW ... FORCE will also fail if the view definition names a constraint that does not exist.

NO FORCE

Specify NOFORCE if you want to create the view only if the base tables exist and the owner of the schema containing the view has privileges on them. This is the default.

schema

Specify the schema to contain the view. If you omit schema, then Oracle Database creates the view in your own schema.

view

Specify the name of the view or the object view.

Restriction on Views

If a view has INSTEAD OF triggers, then any views created on it must have INSTEAD OF triggers, even if the views are inherently updatable.

alias

Specify names for the expressions selected by the defining query of the view. The number of aliases must match the number of expressions selected by the view. Aliases must follow the rules for naming Oracle Database schema objects. Aliases must be unique within the view.

If you omit the aliases, the database derives them from the columns or column aliases in the query. For this reason, you must use aliases if the query contains expressions rather than only column names. Also, you must specify aliases if the view definition includes constraints.

Restriction on View Aliases

You cannot specify an alias when creating an object view.

inline_constraint and out_of_line_constraint

You can specify constraints on views and object views. You define the constraint at the view level using the out_of_line_constraint clause. You define the constraint as part of column or attribute specification using the inline_constraint clause after the appropriate alias.

Oracle Database does not enforce view constraints. For a full discussion of view constraints, including restrictions, please refer to constraint in the documentation on constraints.

object_view_clause

The object_view_clause lets you define a view on an object type.

OF type_name Clause

Use this clause to explicitly create an object view of type type_name. The columns of an object view correspond to the top-level attributes of type type_name. Each row will contain an object instance and each instance will be associated with an object identifier (OID) as specified in the WITH OBJECT IDENTIFIER clause. If you omit schema, then the database creates the object view in your own schema.

Object tables, as well as XMLType tables, object views, and XMLType views, do not have any column names specified for them. Therefore, Oracle Database defines a system-generated pseudocolumn OBJECT_ID. You can use this column name in queries and to create object views with the WITH OBJECT IDENTIFIER clause.

WITH OBJECT IDENTIFIER Clause

Use the WITH OBJECT IDENTIFIER clause to specify a top-level (root) object view. This clause lets you specify the attributes of the object type that will be used as a key to identify each row in the object view. In most cases these attributes correspond to the primary key columns of the base table. You must ensure that the attribute list is unique and identifies exactly one row in the view.

Restrictions on Object Views

• If you try to dereference or pin a primary key REF that resolves to more than one instance in the object view, then the database returns an error.

• You cannot specify this clause if you are creating a subview, because subviews inherit object identifiers from superviews.

If the object view is defined on an object table or an object view, then you can omit this clause or specify DEFAULT.

DEFAULT

Specify DEFAULT if you want the database to use the intrinsic object identifier of the underlying object table or object view to uniquely identify each row.

attribute

For attribute, specify an attribute of the object type from which the database should create the object identifier for the object view.

UNDER Clause

Use the UNDER clause to specify a subview based on an object superview.

Restrictions on Subviews

• You must create a subview in the same schema as the superview.

• The object type type_name must be the immediate subtype of superview.

• You can create only one subview of a particular type under the same superview.

AS subquery

Specify a subquery that identifies columns and rows of the table(s) that the view is based on. The select list of the subquery can contain up to 1000 expressions.

If you create views that refer to remote tables and views, then the database links you specify must have been created using the CONNECT TO clause of the CREATE DATABASE LINK statement, and you must qualify them with a schema name in the view subquery.

If you create a view with the flashback_query_clause in the defining query, the database does not interpret the AS OF expression at create time but rather each time a user subsequently queries the view.

Restrictions on the Defining Query of a View

• The subquery cannot select the CURRVAL or NEXTVAL pseudocolumns.

• If the subquery selects the ROWID, ROWNUM, or LEVEL pseudocolumns, then those columns must have aliases in the view subquery.

• If the subquery uses an asterisk (*) to select all columns of a table, and you later add new columns to the table, then the view will not contain those columns until you re-create the view by issuing a CREATE OR REPLACE VIEW statement.

• For object views, the number of elements in the subquery select list must be the same as the number of top-level attributes for the object type. The datatype of each of the selecting elements must be the same as the corresponding top-level attribute.

• You cannot specify the SAMPLE clause.

The preceding restrictions apply to materialized views as well.

Notes on Updatable Views

An updatable view is one you can use to insert, update, or delete base table rows. You can create a view to be inherently updatable, or you can create an INSTEAD OF trigger on any view to make it updatable.

To learn whether and in what ways the columns of an inherently updatable view can be modified, query the USER_UPDATABLE_COLUMNS data dictionary view. The information displayed by this view is meaningful only for inherently updatable views. For a view to be inherently updatable, the following conditions must be met:

• Each column in the view must map to a column of a single table. For example, if a view column maps to the output of a TABLE clause (an unnested collection), then the view is not inherently updatable.

• The view must not contain any of the following constructs:

A set operator

a DISTINCT operator

An aggregate or analytic function

A GROUP BY, ORDER BY, MODEL, CONNECT BY, or START WITH clause

A collection expression in a SELECT list

A subquery in a SELECT list

A subquery designated WITH READ ONLY

Joins, with some exceptions, as documented in Oracle Database Administrator's Guide

• In addition, if an inherently updatable view contains pseudocolumns or expressions, then you cannot update base table rows with an UPDATE statement that refers to any of these pseudocolumns or expressions.

• If you want a join view to be updatable, then all of the following conditions must be true:

  • The DML statement must affect only one table underlying the join.

  • For an INSERT statement, the view must not be created WITH CHECK OPTION, and all columns into which values are inserted must come from a key-preserved table. A key-preserved table is one for which every primary key or unique key value in the base table is also unique in the join view.

  • For an UPDATE statement, all columns updated must be extracted from a key-preserved table. If the view was created WITH CHECK OPTION, then join columns and columns taken from tables that are referenced more than once in the view must be shielded from UPDATE.

• For a DELETE statement, if the join results in more than one key-preserved table, then Oracle Database deletes from the first table named in the FROM clause, whether or not the view was created WITH CHECK OPTION.

An updatable view is one you can use to insert, update, or delete base table rows. You can create a view to be inherently updatable, or you can create an INSTEAD OF trigger on any view to make it updatable.

XMLType_view_clause

Use this clause to create an XMLType view, which displays data from an XMLSchema-based table of type XMLType. The XMLSchema_spec indicates the XMLSchema to be used to map the XML data to its object-relational equivalents. The XMLSchema must already have been created before you can create an XMLType view.

Object tables, as well as XMLType tables, object views, and XMLType views, do not have any column names specified for them. Therefore, Oracle Database defines a system-generated pseudocolumn OBJECT_ID. You can use this column name in queries and to create object views with the WITH OBJECT IDENTIFIER clause.

subquery_restriction_clause

Use the subquery_restriction_clause to restrict the defining query of the view in one of the following ways:

WITH READ ONLY

Specify WITH READ ONLY to indicate that the table or view cannot be updated.

WITH CHECK OPTION

Specify WITH CHECK OPTION to indicate that Oracle Database prohibits any changes to the table or view that would produce rows that are not included in the subquery. When used in the subquery of a DML statement, you can specify this clause in a subquery in the FROM clause but not in subquery in the WHERE clause.

CONSTRAINT constraint

Specify the name of the CHECK OPTION constraint. If you omit this identifier, then Oracle automatically assigns the constraint a name of the form SYS_Cn, where n is an integer that makes the constraint name unique within the database.

WITH READ ONLY

Specify WITH READ ONLY to indicate that the table or view cannot be updated.

WITH CHECK OPTION

Specify WITH CHECK OPTION to indicate that Oracle Database prohibits any changes to the table or view that would produce rows that are not included in the subquery. When used in the subquery of a DML statement, you can specify this clause in a subquery in the FROM clause but not in subquery in the WHERE clause.

CONSTRAINT constraint

Specify the name of the CHECK OPTION constraint. If you omit this identifier, then the database automatically assigns the constraint a name of the form SYS_Cn, where n is an integer that makes the constraint name unique within the database.

Examples

Creating a View: Example

The following statement creates a view of the sample table employees named emp_view. The view shows the employees in department 20 and their annual salary:

CREATE VIEW emp_view AS 
   SELECT last_name, salary*12 annual_salary
   FROM employees 
   WHERE department_id = 20;

The view declaration need not define a name for the column based on the expression salary*12, because the subquery uses a column alias (annual_salary) for this expression.

Creating a View with Constraints: Example

The following statement creates a restricted view of the sample table hr.employees and defines a unique constraint on the email view column and a primary key constraint for the view on the emp_id view column:

CREATE VIEW emp_sal (emp_id, last_name, 
      email UNIQUE RELY DISABLE NOVALIDATE,
   CONSTRAINT id_pk PRIMARY KEY (emp_id) RELY DISABLE NOVALIDATE)
   AS SELECT employee_id, last_name, email FROM employees;

Creating an Updatable View: Example

The following statement creates an updatable view named clerk of all clerks in the employees table. Only the employees' IDs, last names, department numbers, and jobs are visible in this view, and these columns can be updated only in rows where the employee is a kind of clerk:

CREATE VIEW clerk AS
   SELECT employee_id, last_name, department_id, job_id 
   FROM employees
   WHERE job_id = 'PU_CLERK' 
      or job_id = 'SH_CLERK' 
      or job_id = 'ST_CLERK';

This view lets you change the job_id of a purchasing clerk to purchasing manager (PU_MAN):

UPDATE clerk SET job_id = 'PU_MAN' WHERE employee_id = 118;

The next example creates the same view WITH CHECK OPTION. You cannot subsequently insert a new row into clerk if the new employee is not a clerk. You can update an employee's job_id from one type of clerk to another type of clerk, but the update in the preceding statement would fail, because the view cannot access employees with non-clerk job_id.

CREATE VIEW clerk AS
   SELECT employee_id, last_name, department_id, job_id 
   FROM employees
   WHERE job_id = 'PU_CLERK' 
      or job_id = 'SH_CLERK' 
      or job_id = 'ST_CLERK'
   WITH CHECK OPTION;

Creating a Join View: Example

A join view is one whose view subquery contains a join. If at least one column in the join has a unique index, then it may be possible to modify one base table in a join view. You can query USER_UPDATABLE_COLUMNS to see whether the columns in a join view are updatable. For example:

CREATE VIEW locations_view AS
   SELECT d.department_id, d.department_name, l.location_id, l.city
   FROM departments d, locations l
   WHERE d.location_id = l.location_id;

SELECT column_name, updatable 
   FROM user_updatable_columns
   WHERE table_name = 'LOCATIONS_VIEW';

COLUMN_NAME                    UPD
------------------------------ ---
DEPARTMENT_ID                  YES
DEPARTMENT_NAME                YES
LOCATION_ID                    NO
CITY                           NO

In the preceding example, the primary key index on the location_id column of the locations table is not unique in the locations_view view. Therefore, locations is not a key-preserved table and columns from that base table are not updatable.

INSERT INTO locations_view VALUES
   (999, 'Entertainment', 87, 'Roma');
INSERT INTO locations_view VALUES
*
ERROR at line 1:
ORA-01776: cannot modify more than one base table through a join view

You can insert, update, or delete a row from the departments base table, because all the columns in the view mapping to the departments table are marked as updatable and because the primary key of departments is retained in the view.

INSERT INTO locations_view (department_id, department_name)
   VALUES (999, 'Entertainment');

1 row created.

Creating a Read-Only View: Example

The following statement creates a read-only view named customer_ro of the oe.customers table. Only the customers' last names, language, and credit limit are visible in this view:

CREATE VIEW customer_ro (name, language, credit)
      AS SELECT cust_last_name, nls_language, credit_limit
      FROM customers
      WITH READ ONLY;

Creating an Object View: Example

The following example shows the creation of the type inventory_typ in the oc schema, and the oc_inventories view that is based on that type:

CREATE TYPE inventory_typ AS OBJECT
    ( product_id          number(6) 
    , warehouse           warehouse_typ
    , quantity_on_hand    number(8)
    ) ;

CREATE OR REPLACE VIEW oc_inventories OF inventory_typ 
   WITH OBJECT IDENTIFIER (product_id)
   AS SELECT i.product_id, 
     warehouse_typ(w.warehouse_id, w.warehouse_name, w.location_id),
     i.quantity_on_hand
   FROM inventories i, warehouses w
   WHERE i.warehouse_id=w.warehouse_id;

Creating a View on an XMLType Table: Example

The following example builds a regular view on the XMLType table xwarehouses, which was created in "Examples ":

CREATE VIEW warehouse_view AS
   SELECT VALUE(p) AS warehouse_xml
   FROM xwarehouses p;

You select from such a view as follows:

SELECT e.warehouse_xml.getclobval()
   FROM warehouse_view e
   WHERE EXISTSNODE(warehouse_xml, '//Docks') =1;

Creating an XMLType View: Example

In some cases you may have an object-relational table upon which you would like to build an XMLType view. The following example creates an object-relational table resembling the XMLType column warehouse_spec in the sample table oe.warehouses, and then creates an XMLType view of that table:

CREATE TABLE warehouse_table 
(
  WarehouseID       NUMBER,
  Area              NUMBER,
  Docks             NUMBER,
  DockType          VARCHAR2(100),
  WaterAccess       VARCHAR2(10),
  RailAccess        VARCHAR2(10),
  Parking           VARCHAR2(20),
  VClearance        NUMBER
);

INSERT INTO warehouse_table 
   VALUES(5, 103000,3,'Side Load','false','true','Lot',15);

CREATE VIEW warehouse_view OF XMLTYPE
 XMLSCHEMA "http://www.oracle.com/xwarehouses.xsd" 
    ELEMENT "Warehouse"
    WITH OBJECT ID 
    (extract(OBJECT_VALUE, '/Warehouse/Area/text()').getnumberval())
 AS SELECT XMLELEMENT("Warehouse",
            XMLFOREST(WarehouseID as "Building",
                      area as "Area",
                      docks as "Docks",
                      docktype as "DockType",
                      wateraccess as "WaterAccess",
                      railaccess as "RailAccess",
                      parking as "Parking",
                      VClearance as "VClearance"))
  FROM warehouse_table;

You query this view as follows:

SELECT VALUE(e) FROM warehouse_view e;