|Oracle8i Administrator's Guide
Release 2 (8.1.6)
This chapter describes aspects of view management, and includes the following topics:
Before attempting tasks described in this chapter, familiarize yourself with the concepts in Chapter 12, "Guidelines for Managing Schema Objects".
A view is a tailored presentation of the data contained in one or more tables (or other views), and takes the output of a query and treats it as a table. You can think of a view as a "stored query" or a "virtual table." You can use views in most places where a table can be used.
This section describes aspects of managing views, and includes the following topics:
To create a view, you must fulfill the requirements listed below.
You can create views using the CREATE VIEW statement. Each view is defined by a query that references tables, snapshots, or other views. As with all subqueries, the query that defines a view cannot contain the FOR UPDATE clause.
The following statement creates a view on a subset of data in the EMP table:
CREATE VIEW sales_staff AS SELECT empno, ename, deptno FROM emp WHERE deptno = 10 WITH CHECK OPTION CONSTRAINT sales_staff_cnst;
The query that defines the SALES_STAFF view references only rows in department 10. Furthermore, the CHECK OPTION creates the view with the constraint (named SALES_STAFF_CNST) that INSERT and UPDATE statements issued against the view cannot result in rows that the view cannot select. For example, the following INSERT statement successfully inserts a row into the EMP table by means of the SALES_STAFF view, which contains all rows with department number 10:
However, the following INSERT statement is rolled back and returns an error because it attempts to insert a row for department number 30, which cannot be selected using the SALES_STAFF view:
The view could optionally have been constructed specifying the WITH READ ONLY clause, which prevents any updates, inserts, or deletes from being done to the base table through the view. If no WITH clause is specified, the view, with some restrictions, is inherently updatable.
The restrictions for creating views are included in the description of the CREATE VIEW statement in the Oracle8i SQL Reference. Please refer to that book for specifics.
For detailed syntax, restriction, and authorization information relating to creating or replacing, updating, altering, and dropping views, see Oracle8i SQL Reference.
You can also create views that specify more than one base table or view in the FROM clause. These are called join views. The following statement creates the DIVISION1_STAFF view that joins data from the EMP and DEPT tables:
CREATE VIEW division1_staff AS SELECT ename, empno, job, dname FROM emp, dept WHERE emp.deptno IN (10, 30) AND emp.deptno = dept.deptno;
An updatable join view is a join view where UPDATE, INSERT, and DELETE operations are allowed. See "Updating a Join View" for further discussion.
In accordance with the ANSI/ISO standard, Oracle expands any wildcard in a top-level view query into a column list when a view is created and stores the resulting query in the data dictionary; any subqueries are left intact. The column names in an expanded column list are enclosed in quote marks to account for the possibility that the columns of the base object were originally entered with quotes and require them for the query to be syntactically correct.
As an example, assume that the DEPT view is created as follows:
Oracle stores the defining query of the DEPT view as:
Views created with errors do not have wildcards expanded. However, if the view is eventually compiled without errors, wildcards in the defining query are expanded.
If there are no syntax errors in a CREATE VIEW statement, Oracle can create the view even if the defining query of the view cannot be executed; the view is considered "created with errors." For example, when a view is created that refers to a nonexistent table or an invalid column of an existing table, or when the view owner does not have the required privileges, the view can be created anyway and entered into the data dictionary. However, the view is not yet usable.
To create a view with errors, you must include the FORCE option of the CREATE VIEW statement.
By default, views with errors are not created as VALID. When you try to create such a view, Oracle returns a message indicating the view was created with errors. The status of a view created with errors is INVALID. If conditions later change so that the query of an invalid view can be executed, the view can be recompiled and be made valid (usable). For information changing conditions and their impact on views, see "Managing Object Dependencies".
An updatable join view (also referred to as a modifiable join view) is a view that contains more than one table in the top-level FROM clause of the SELECT statement, and is not restricted by the WITH READ ONLY clause.
There are some restrictions and conditions which may affect whether a join view is updatable. These are listed in the description of the CREATE VIEW statement in the Oracle8i SQL Reference. Please refer to that book for specifics.
Additionally, if a view is a join on other nested views, then the other nested views must be mergeable into the top level view. For a discussion of mergeable and unmergeable views, and more generally, how the optimizer optimizes statements reverencing views, see Oracle8i Concepts and Oracle8i Designing and Tuning for Performance.
There are data dictionary views which indicate whether the columns in a join view are updatable. See Table 18-1, "UPDATABLE_COLUMNS Views" for descriptions of these views.
The rules for updatable join views are as follows:
Any INSERT, UPDATE, or DELETE operation on a join view can modify only one underlying base table at a time.
All updatable columns of a join view must map to columns of a key-preserved table. If the view is defined with the WITH CHECK OPTION clause, then all join columns and all columns of repeated tables are non-updatable.
Rows from a join view can be deleted as long as there is exactly one key-preserved table in the join. If the view is defined with the WITH CHECK OPTION clause and the key preserved table is repeated, then the rows cannot be deleted from the view.
An INSERT statement must not explicitly or implicitly refer to the columns of a non-key preserved table. If the join view is defined with the WITH CHECK OPTION clause, INSERT statements are not permitted.
Examples illustrating these rules, and a discussion of key-preserved tables, are presented in succeeding sections.
The examples given work only if you explicitly define the primary and foreign keys in the tables, or define unique indexes. Following are the appropriately constrained table definitions for EMP and DEPT.
CREATE TABLE dept ( deptno NUMBER(4) PRIMARY KEY, dname VARCHAR2(14), loc VARCHAR2(13)); CREATE TABLE emp ( empno NUMBER(4) PRIMARY KEY, ename VARCHAR2(10), job VARCHAR2(9), mgr NUMBER(4), sal NUMBER(7,2), comm NUMBER(7,2), deptno NUMBER(2), FOREIGN KEY (DEPTNO) REFERENCES DEPT(DEPTNO));
You could also omit the primary and foreign key constraints listed above, and create a UNIQUE INDEX on DEPT (DEPTNO) to make the following examples work.
The following statement created the EMP_DEPT join view which is referenced in the examples:
CREATE VIEW emp_dept AS SELECT emp.empno, emp.ename, emp.deptno, emp.sal, dept.dname, dept.loc FROM emp, dept WHERE emp.deptno = dept.deptno AND dept.loc IN ('DALLAS', 'NEW YORK', 'BOSTON');
The concept of a key-preserved table is fundamental to understanding the restrictions on modifying join views. A table is key preserved if every key of the table can also be a key of the result of the join. So, a key-preserved table has its keys preserved through a join.
The key-preserving property of a table does not depend on the actual data in the table. It is, rather, a property of its schema. For example, if in the EMP table there was at most one employee in each department, then DEPTNO would be unique in the result of a join of EMP and DEPT, but DEPT would still not be a key-preserved table.
If you SELECT all rows from EMP_DEPT, the results are:
EMPNO ENAME DEPTNO DNAME LOC ---------- ---------- ------- -------------- ----------- 7782 CLARK 10 ACCOUNTING NEW YORK 7839 KING 10 ACCOUNTING NEW YORK 7934 MILLER 10 ACCOUNTING NEW YORK 7369 SMITH 20 RESEARCH DALLAS 7876 ADAMS 20 RESEARCH DALLAS 7902 FORD 20 RESEARCH DALLAS 7788 SCOTT 20 RESEARCH DALLAS 7566 JONES 20 RESEARCH DALLAS 8 rows selected.
In this view, EMP is a key-preserved table, because EMPNO is a key of the EMP table, and also a key of the result of the join. DEPT is not a key-preserved table, because although DEPTNO is a key of the DEPT table, it is not a key of the join.
The general rule is that any UPDATE, INSERT, or DELETE statement on a join view can modify only one underlying base table. The following examples will illustrate rules specific to UPDATE, DELETE, and INSERT statements.
The following example shows an UPDATE statement that successfully modifies the EMP_DEPT view:
The following UPDATE statement would be disallowed on the EMP_DEPT view:
This statement fails with an ORA-01779 error (''cannot modify a column which maps to a non key-preserved table''), because it attempts to modify the base DEPT table, and the DEPT table is not key preserved in the EMP_DEPT view.
In general, all updatable columns of a join view must map to columns of a key-preserved table. If the view is defined using the WITH CHECK OPTION clause, then all join columns and all columns of repeated tables are not modifiable.
So, for example, if the EMP_DEPT view were defined using WITH CHECK OPTION, the following UPDATE statement would fail:
The statement fails because it is trying to update a join column.
You can delete from a join view provided there is one and only one key-preserved table in the join.
The following DELETE statement works on the EMP_DEPT view:
This DELETE statement on the EMP_DEPT view is legal because it can be translated to a DELETE operation on the base EMP table, and because the EMP table is the only key-preserved table in the join.
If you were to create the following view, a DELETE operation could not be performed on the view because both E1 and E2 are key-preserved tables:
CREATE VIEW emp_emp AS SELECT e1.ename, e2.empno, deptno FROM emp e1, emp e2 WHERE e1.empno = e2.empno;
If a view is defined using the WITH CHECK OPTION clause and the key-preserved table is repeated, then rows cannot be deleted from such a view.
CREATE VIEW emp_mgr AS SELECT e1.ename, e2.ename mname FROM emp e1, emp e2 WHERE e1.mgr = e2.empno WITH CHECK OPTION;
No deletion can be performed on this view because the view involves a self-join of the table that is key preserved.
The following INSERT statement on the EMP_DEPT view succeeds:
This statement works because only one key-preserved base table is being modified (EMP), and 40 is a valid DEPTNO in the DEPT table (thus satisfying the FOREIGN KEY integrity constraint on the EMP table).
An INSERT statement like the following would fail for the same reason that such an UPDATE on the base EMP table would fail: the FOREIGN KEY integrity constraint on the EMP table is violated.
The following INSERT statement would fail with an ORA-01776 error (''cannot modify more than one base table through a view'').
An INSERT cannot implicitly or explicitly refer to columns of a non-key-preserved table. If the join view is defined using the WITH CHECK OPTION clause, then you cannot perform an INSERT to it.
The views described in Table 18-1 can assist you when modifying join views.
Shows all columns in all tables and views in the user's schema that are modifiable.
Shows all columns in all tables and views in the DBA schema that are modifiable.
Shows all columns in all tables and views that are modifiable.
The updatable columns in view EMP_DEPT are shown below.
SELECT column_name, updatable FROM user_updatable_columns WHERE table_name = 'EMP_DEPT'; COLUMN_NAME UPD ------------------------------ --- EMPNO YES ENAME YES DEPTNO YES SAL YES DNAME NO LOC NO 6 rows selected.
You use the ALTER VIEW statement only to explicitly recompile a view that is invalid. If you want to change the definition of a view, see Replacing Views.
The ALTER VIEW statement allows you to locate recompilation errors before run time. You may want to explicitly recompile a view after altering one of its base tables to ensure that the alteration does not affect the view or other objects that depend on it.
To use the ALTER VIEW statement, the view must be in your schema, or you must have the ALTER ANY TABLE system privilege.
You can drop any view contained in your schema. To drop a view in another user's schema, you must have the DROP ANY VIEW system privilege. Drop a view using the DROP VIEW statement. For example, the following statement drops the EMP_DEPT view:
To replace a view, you must have all the privileges required to drop and create a view. If the definition of a view must change, the view must be replaced; you cannot change the definition of a view. You can replace views in the following ways:
Before replacing a view, consider the following effects:
Sequences are database objects from which multiple users may generate unique integers. You can use sequences to automatically generate primary key values. This section describes various aspects of managing sequences, and includes the following topics:
To create a sequence in your schema, you must have the CREATE SEQUENCE system privilege; to create a sequence in another user's schema, you must have the CREATE ANY SEQUENCE privilege.
Create a sequence using the CREATE SEQUENCE statement. For example, the following statement creates a sequence used to generate employee numbers for the EMPNO column of the EMP table:
The CACHE option pre-allocates a set of sequence numbers and keeps them in memory so that sequence numbers can be accessed faster. When the last of the sequence numbers in the cache has been used, Oracle reads another set of numbers into the cache.
Oracle might skip sequence numbers if you choose to cache a set of sequence numbers. For example, when an instance abnormally shuts down (for example, when an instance failure occurs or a SHUTDOWN ABORT statement is issued), sequence numbers that have been cached but not used are lost. Also, sequence numbers that have been used but not saved are lost as well. Oracle might also skip cached sequence numbers after an export and import; see Oracle8i Utilities for details.
For information about how the Oracle Parallel Server affects cached sequence numbers, see Oracle8i Parallel Server Administration, Deployment, and Performance.
For performance information on caching sequence numbers, see Oracle8i Designing and Tuning for Performance.
To alter a sequence, your schema must contain the sequence, or you must have the ALTER ANY SEQUENCE system privilege. You can alter a sequence to change any of the parameters that define how it generates sequence numbers except the sequence's starting number. To change the starting point of a sequence, drop the sequence and then re-create it. When you perform DDL on sequence numbers you will lose the cache values.
Alter a sequence using the ALTER SEQUENCE statement. For example, the following statement alters the EMP_SEQUENCE:
You can drop any sequence in your schema. To drop a sequence in another schema, you must have the DROP ANY SEQUENCE system privilege. If a sequence is no longer required, you can drop the sequence using the DROP SEQUENCE statement. For example, the following statement drops the ORDER_SEQ sequence:
When a sequence is dropped, its definition is removed from the data dictionary. Any synonyms for the sequence remain, but return an error when referenced.
A synonym is an alias for a schema object. Synonyms can provide a level of security by masking the name and owner of an object and by providing location transparency for remote objects of a distributed database. Also, they are convenient to use and reduce the complexity of SQL statements for database users.
Synonyms allow underlying objects to be renamed or moved, where only the synonym needs to be redefined and applications based on the synonym continue to function without modification.
You can create both public and private synonyms. A public synonym is owned by the special user group named PUBLIC and is accessible to every user in a database. A private synonym is contained in the schema of a specific user and available only to the user and the user's grantees.
This section includes the following synonym management information:
To create a private synonym in your own schema, you must have the CREATE SYNONYM privilege; to create a private synonym in another user's schema, you must have the CREATE ANY SYNONYM privilege. To create a public synonym, you must have the CREATE PUBLIC SYNONYM system privilege.
Create a synonym using the CREATE SYNONYM statement. The underlying schema object need not exist. The following statement creates a public synonym named PUBLIC_EMP on the EMP table contained in the schema of JWARD:
You can drop any private synonym in your own schema. To drop a private synonym in another user's schema, you must have the DROP ANY SYNONYM system privilege. To drop a public synonym, you must have the DROP PUBLIC SYNONYM system privilege.
Drop a synonym that is no longer required using DROP SYNONYM statement. To drop a private synonym, omit the PUBLIC keyword; to drop a public synonym, include the PUBLIC keyword.
For example, the following statement drops the private synonym named EMP:
The following statement drops the public synonym named PUBLIC_EMP:
When you drop a synonym, its definition is removed from the data dictionary. All objects that reference a dropped synonym remain; however, they become invalid (not usable). For more information about how dropping synonyms can affect other schema objects, see "Managing Object Dependencies"