Les12 creating views

Editor's Notes

• #2: Schedule:TimingTopic 20 minutesLecture 20 minutesPractice 40 minutesTotal
• #3: Lesson Aim In this lesson, you will learn to create and use views. You will also learn to query the relevant data dictionary object to retrieve information about views.
• #5: What Is a View? You can present logical subsets or combinations of data by creating views of tables. A view is a logical table based on a table or another view. A view contains no data of its own but is like a window through which data from tables can be viewed or changed. The tables on which a view is based are called base tables. The view is stored as a SELECT statement in the data dictionary.
• #6: Advantages of Views Views restrict access to the database because the view can display a selective portion of the database. Views allow users to make simple queries to retrieve the results from complicated queries. For example, views allow users to query information from multiple tables without knowing how to write a join statement. Views provide data independence for ad hoc users and application programs. One view can be used to retrieve data from several tables. Views provide groups of users access to data according to their particular criteria. For more information, seeOracle Server SQL Reference, Release 8, “CREATE VIEW.”
• #7: Simple Views Versus Complex Views There are two classifications for views: simple and complex. The basic difference is related to the DML (insert, update, and delete) operations. A simple view is one that: Derives data from only one table Contains no functions or groups of data Can perform DML through the view A complex view is the one that: Derives data from many tables Contains functions or groups of data Does not always allow DML through the view
• #8: Creating a View You can create a view by embedding a subquery within the CREATE VIEW statement. In the syntax: OR REPLACEre-creates the view if it already exists FORCEcreates the view regardless of whether or not the base tables exist NOFORCEcreates the view only if the base tables exist (This is the default.) viewis the name of the view aliasspecifies names for the expressions selected by the view’s query (The number of aliases must match the number of expressions selected by the view.) subqueryis a complete SELECT statement (You can use aliases for the columns in the SELECT list.) WITH CHECK OPTIONspecifies that only rows accessible to the view can be inserted or updated constraintis the name assigned to the CHECK OPTION constraint WITH READ ONLYensures that no DML operations can be performed on this view
• #9: Creating a View (continued) The example on the slide creates a view that contains the employee number, name, and job title for all the employees in department 10. You can display the structure of the view by using the SQL*Plus DESCRIBE command. Name Null? Type ------------------------------- -------- ------------ EMPNO NOT NULL NUMBER(4) ENAME VARCHAR2(10) JOB VARCHAR2(9) Guidelines for creating a view: The subquery that defines a view can contain complex SELECT syntax, including joins, groups, and subqueries. The subquery that defines the view cannot contain an ORDER BY clause. The ORDER BY clause is specified when you retrieve data from the view. If you do not specify a constraint name for a view created with the CHECK OPTION, the system will assign a default name in the format SYS_Cn. You can use the OR REPLACE option to change the definition of the view without dropping and re-creating it or regranting object privileges previously granted on it.
• #10: Creating a View (continued) You can control the column names by including column aliases within the subquery. The example on the slide creates a view containing the employee number (empno) with the alias EMPLOYEE_NUMBER, name (ename) with the alias NAME, and salary (sal) with the alias SALARY for department 30. Alternatively, you can control the column names by including column aliases in the CREATE VIEW clause.
• #11: Retrieving Data from a View You can retrieve data from a view as you would from any table. You can either display the contents of the entire view or just view specific rows and columns.
• #12: Views in the Data Dictionary
• #13: Modifying a View The OR REPLACE option allows a view to be created even if one exists with this name already, thus replacing the old version of the view for its owner. This means that the view can be altered without dropping, re-creating, and regranting object privileges. Note: When assigning column aliases in the CREATE VIEW clause, remember that the aliases are listed in the same order as the columns in the subquery. Class Management Note The OR REPLACE option started with Oracle7. With earlier versions of Oracle, if the view needed to be changed, it had to be dropped and re-created. Demo: l12emp.sql Purpose: To illustrate creating a view using aliases.
• #14: Creating a Complex View The example on the slide creates a complex view of the department names, minimum salary, maximum salary, and average salary by the department. Note that alternative names have been specified for the view. This is a requirement if any column of the view is derived from a function or an expression. You can view the structure of the view by using the SQL*Plus DESCRIBE command. Display the contents of the view by issuing a SELECT statement.
• #15: Performing DML Operations on a View You can perform DML operations on data through a view if those operations follow certain rules. You can remove a row from a view unless it contains any of the following: Group functions A GROUP BY clause The DISTINCT keyword
• #16: Performing DML Operations on a View (continued) You can modify data in a view unless it contains any of the conditions mentioned in the previous slide and any of the following: Columns defined by expressions—for example, SALARY * 12 The ROWNUM pseudocolumn You can add data through a view unless it contains any of the above and there are NOT NULL columns, without a default value, in the base table that are not selected by the view. All required values must be present in the view. Remember that you are adding values directly into the underlying table through the view. For more information, see 0racle8 Server SQL Reference, Release 8, “CREATE VIEW.” Class Management Note Oracle7.3 and Oracle8 allow you, with some restrictions, to modify views that involve joins. The restrictions for DML operations described above also apply to join views. Any UPDATE, INSERT, or DELETE statement on a join view can modify only one underlying base table. If at least one column in the subquery 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 Page 12-16) If the user does not supply a constraint name, the system assigns a name in the form SYS_Cn, where n is an integer that makes the constraint name unique within the system. Demo: l12vu1.sql, l12vu2.sql Purpose: To illustrate creating views with and without the CHECK OPTION.
• #17: Using the WITH CHECK OPTION Clause It is possible to perform referential integrity checks through views. You can also enforce constraints at the database level. The view can be used to protect data integrity, but the use is very limited. The WITH CHECK OPTION clause specifies that INSERTS and UPDATES performed through the view are not allowed to create rows that the view cannot select, and therefore it allows integrity constraints and data validation checks to be enforced on data being inserted or updated. If there is an attempt to perform DML operations on rows that the view has not selected, an error is displayed, with the constraint name if that has been specified. SQL> UPDATE empvu20 2 SET deptno = 10 3 WHERE empno = 7788; update empvu20 * ERROR at line 1: ORA-01402: view WITH CHECK OPTION where-clause violation Note: No rows are updated because if the department number were to change to 10, the view would no longer be able to see that employee. Therefore, with the WITH CHECK OPTION clause, the view can see only department 20 employees and does not allow the department number for those employees to be changed through the view.
• #18: Denying DML Operations You can ensure that no DML operations occur on your view by creating it with the WITH READ ONLY option. The example on the slide modifies the EMPVU10 view to prevent any DML operations on the view. Any attempts to remove a row from the view will result in an error. Any attempts to insert a row or modify a row using the view will result in Oracle Server error -01733: virtual column not allowed here.
• #19: Removing a View You use the DROP VIEW statement to remove a view. The statement removes the view definition from the database. Dropping views has no effect on the tables on which the view was based. Views or other applications based on deleted views become invalid. Only the creator or a user with the DROP ANY VIEW privilege can remove a view. In the syntax: viewis the name of the view
• #20: What Is a View? A view is based on a table or another view and acts as a window through which data on tables can be viewed or changed. A view does not contain data. The definition of the view is stored in the data dictionary. You can see the definition of the view in the USER_VIEWS data dictionary table. Advantages of Views Restrict database access Simplify queries Provide data independence Allow multiple views of the same data Remove views without affecting the underlying data View Options Can be a simple view based on one table Can be a complex view based on more than one table or can contain groups or functions Can be replaced if one of the same name exists Contain a check constraint Can be read-only
• #21: Practice Overview In this practice, you will create simple and complex views and attempt to perform DML statements on the views.
• #22: Practice 12 1.Create a view called EMP_VU based on the employee number, employee name, anddepartment number from the EMP table. Change the heading for the employee name toEMPLOYEE. 2.Display the contents of the EMP_VU view. EMPNO EMPLOYEE DEPTNO ----- -------- ------ 7839 KING 10 7698 BLAKE 30 7782 CLARK 10 7566 JONES 20 7654 MARTIN 30 7499 ALLEN 30 7844 TURNER 30 7900 JAMES 30 7521 WARD 30 7902 FORD 20 7369 SMITH 20 7788 SCOTT 20 7876 ADAMS 20 7934 MILLER 10 14 rows selected. 3.Select the view name and text from the data dictionary USER_VIEWS. VIEW_NAME TEXT ----------- ---------------------------------------- EMP_VU SELECT empno, ename employee, deptno FROM emp 4.Using your view EMP_VU, enter a query to display all employee names and department numbers. EMPLOYEE DEPTNO ---------- --------- KING 10 BLAKE 30 CLARK 10 JONES 20 MARTIN 30 ... 14 rows selected.
• #23: Practice 12 (continued) 5.Create a view named DEPT20 that contains the employee number, employee name, anddepartment number for all employees in department 20. Label the view columnEMPLOYEE_ID, EMPLOYEE, and DEPARTMENT_ID. Do not allow an employee to be reassigned to another department through the view. 6.Display the structure and contents of the DEPT20 view. Name Null? Type --------------- -------- ------ EMPLOYEE_ID NOT NULL NUMBER(4) EMPLOYEE VARCHAR2(10) DEPARTMENT_ID NOT NULL NUMBER(2) EMPLOYEE_ID EMPLOYEE DEPARTMENT_ID ----------- -------- ------------- 7566 JONES20 7902 FORD20 7369 SMITH20 7788 SCOTT20 7876 ADAMS20 7.Attempt to reassign Smith to department 30. If you have time, complete the following exercise: 8.Create a view called SALARY_VU based on the employee name, department name, salary, and salary grade for all employees. Label the columns Employee, Department, Salary, andGrade, respectively.