Creating Views

14-2 Database Systems Objectives Explain the concept of a view. Create simple and complex views. Retrieve data through a view. Alter the definition of a view. Insert, Update, Delete data through a view. Drop a view.

14-3 Database Systems ID LAST_NAME FIRST_NAME TITLE DEPT_ID Velasquez Carmen President 50 2 Ngao LaDoris VP, Operations 41 3 Nagayama Midori VP, Sales 31 4 Quick-To-See Mark VP, Finance 10 5 Ropeburn Audry VP, Administration 50 6 Urguhart Molly Warehouse Manager 41 7 Menchu Roberta Warehouse Manager 42 8 Biri Ben Warehouse Manager 43 9 Catchpole Antoinette Warehouse Manager Havel Marta Warehouse Manager Magee Colin Sales Representative Giljum Henry Sales Representative Sedeghi Yasmin Sales Representative Nguyen Mai Sales Representative Dumas Andre Sales Representative Maduro Elena Stock Clerk Smith George Stock Clerk Nozaki Akira Stock Clerk Patel Vikram Stock Clerk Newman Chad Stock Clerk Markarian Alexander Stock Clerk Chang Eddie Stock Clerk Patel Radha Stock Clerk Dancs Bela Stock Clerk Schwartz Sylvie Stock Clerk 45 What Is a View? ID LAST_NAME TITLE Havel Warehouse Manager 24 Dancs Stock Clerk 25 Schwartz Stock Clerk EMP Table EMPVU45 View

14-4 Database Systems Why Use Views? To restrict database access To make complex queries easy To allow data independence To present different views of the same data

14-5 Database Systems Simple Views and Complex Views Number of tables Contain functions Contain groups of data DML through view Complex Views One or more YesYes Not always Simple Views OneNoNoYes

14-6 Database Systems Creating a View: Syntax Embed a subquery within the CREATE VIEW statement. The subquery can contain complex SELECT syntax. The subquery cannot contain an ORDER BY clause. CREATE [OR REPLACE] [FORCE|NOFORCE] VIEW view [(alias[, alias]...)] [(alias[, alias]...)] AS subquery [WITH CHECK OPTION [CONSTRAINT constraint]] [WITH READ ONLY] CREATE [OR REPLACE] [FORCE|NOFORCE] VIEW view [(alias[, alias]...)] [(alias[, alias]...)] AS subquery [WITH CHECK OPTION [CONSTRAINT constraint]] [WITH READ ONLY]

14-7 Database Systems Creating a View: Syntax OR REPLACE – re-create the view if it already exists. FORCE – create the view regardless whether the base table exists or not. WITH CHECK OPTION – specifies that only rows accessible to the view can be inserted or updated. WITH READ ONLY – no DML can be performed. CREATE [OR REPLACE] [FORCE|NOFORCE] VIEW view [(alias[, alias]...)] [(alias[, alias]...)] AS subquery [WITH CHECK OPTION [CONSTRAINT constraint]] [WITH READ ONLY] CREATE [OR REPLACE] [FORCE|NOFORCE] VIEW view [(alias[, alias]...)] [(alias[, alias]...)] AS subquery [WITH CHECK OPTION [CONSTRAINT constraint]] [WITH READ ONLY]

14-8 Database Systems Creating a View: Example Create the EMPVU10 view, which contains the employee number, last name, and job title for employees in department 10. Describe the structure of the view by using the SQL*Plus DESCRIBE command. Display data from the view by entering a SELECT statement against the view. SQL> CREATE VIEW empvu10 2 AS SELECTempno, ename, job 2 AS SELECTempno, ename, job 3 FROMemp 3 FROMemp 4 WHEREdeptno = 10; 4 WHEREdeptno = 10; View created. SQL> CREATE VIEW empvu10 2 AS SELECTempno, ename, job 2 AS SELECTempno, ename, job 3 FROMemp 3 FROMemp 4 WHEREdeptno = 10; 4 WHEREdeptno = 10; View created.

14-9 Database Systems Creating a View: Example Create a view by using column aliases in the subquery. Select the columns from this view by the given alias name. SQL> CREATE VIEW salvu30 2 AS SELECTempno EMPLOYEE_NUMBER, ename FIRST_NAME, 2 AS SELECTempno EMPLOYEE_NUMBER, ename FIRST_NAME, 3 sal MONTHLY_SALARY 3 sal MONTHLY_SALARY 4 FROMemp 4 FROMemp 5 WHEREdeptno = 30; 5 WHEREdeptno = 30; View created. SQL> CREATE VIEW salvu30 2 AS SELECTempno EMPLOYEE_NUMBER, ename FIRST_NAME, 2 AS SELECTempno EMPLOYEE_NUMBER, ename FIRST_NAME, 3 sal MONTHLY_SALARY 3 sal MONTHLY_SALARY 4 FROMemp 4 FROMemp 5 WHEREdeptno = 30; 5 WHEREdeptno = 30; View created.

14-10 Database Systems Modifying a View: Example Modify the EMPVU10 view by using CREATE OR REPLACE. Add an alias for each column name. Column aliases in the CREATE VIEW clause are listed in the same order as the columns in the subquery. SQL> CREATE OR REPLACE VIEW empvu10 2 (id_number, employee, title) 2 (id_number, employee, title) 3 AS SELECTempno, ename, job 3 AS SELECTempno, ename, job 4 FROMemp 4 FROMemp 5 WHEREdeptno = 10; 5 WHEREdeptno = 10; View created. SQL> CREATE OR REPLACE VIEW empvu10 2 (id_number, employee, title) 2 (id_number, employee, title) 3 AS SELECTempno, ename, job 3 AS SELECTempno, ename, job 4 FROMemp 4 FROMemp 5 WHEREdeptno = 10; 5 WHEREdeptno = 10; View created.

14-11 Database Systems Creating a Complex View: Example Create a complex view that contains group functions to display values from two tables. SQL> CREATE VIEW dept_sum_vu 2 (name, minsal, maxsal, avgsal) 2 (name, minsal, maxsal, avgsal) 3 AS SELECTd.dname, MIN(e.sal), 3 AS SELECTd.dname, MIN(e.sal), 4 MAX(e.sal), AVG(e.sal) 4 MAX(e.sal), AVG(e.sal) 5 FROMemp e, dept d 5 FROMemp e, dept d 6 WHEREe.deptno = d.deptno 6 WHEREe.deptno = d.deptno 7 GROUP BY d.dname; 7 GROUP BY d.dname; View created. SQL> CREATE VIEW dept_sum_vu 2 (name, minsal, maxsal, avgsal) 2 (name, minsal, maxsal, avgsal) 3 AS SELECTd.dname, MIN(e.sal), 3 AS SELECTd.dname, MIN(e.sal), 4 MAX(e.sal), AVG(e.sal) 4 MAX(e.sal), AVG(e.sal) 5 FROMemp e, dept d 5 FROMemp e, dept d 6 WHEREe.deptno = d.deptno 6 WHEREe.deptno = d.deptno 7 GROUP BY d.dname; 7 GROUP BY d.dname; View created.

14-12 Database Systems Rules for Performing DML Operations on a View You can perform DML operations on simple views. You cannot remove a row if the view contains – Group functions. – A GROUP BY clause. – The DISTINCT keyword.

14-13 Database Systems Rules for Performing DML Operations on a View continued You cannot modify data in a view if it contains – Any of the above conditions. – Columns defined by expressions. – The ROWNUM pseudocolumn. You cannot add data if the view contains – Any of the above conditions. – Any NOT NULL columns in the base table not selected by the view.

14-14 Database Systems Using the WITH CHECK OPTION Clause Ensure that DML on the view stays within the domain of the view. If you attempt to change the department number for any rows in the view, the statement will fail because it violates the CHECK OPTION constraint. SQL> CREATE OR REPLACE VIEW empvu20 2 AS SELECT* 2 AS SELECT* 3 FROMemp 3 FROMemp 4 WHEREdeptno = 20 4 WHEREdeptno = 20 5 WITH CHECK OPTION CONSTRAINT empvu20_ck; 5 WITH CHECK OPTION CONSTRAINT empvu20_ck; View created. SQL> CREATE OR REPLACE VIEW empvu20 2 AS SELECT* 2 AS SELECT* 3 FROMemp 3 FROMemp 4 WHEREdeptno = 20 4 WHEREdeptno = 20 5 WITH CHECK OPTION CONSTRAINT empvu20_ck; 5 WITH CHECK OPTION CONSTRAINT empvu20_ck; View created.

14-15 Database Systems Using the WITH CHECK OPTION Clause Without check option, employee with empno = 7788 will be removed from the view. SQL> UPDATE empvu20 2 SET deptno = 10 2 SET deptno = 10 3 WHERE empno = 7788; 3 WHERE empno = 7788; Update empvu20 * ERROR at line 1: ORA-01402: view WITH CHECK OPTION where-clause violation SQL> UPDATE empvu20 2 SET deptno = 10 2 SET deptno = 10 3 WHERE empno = 7788; 3 WHERE empno = 7788; Update empvu20 * ERROR at line 1: ORA-01402: view WITH CHECK OPTION where-clause violation

14-16 Database Systems Denying DML Operations Ensure that no DML operations occur by adding the WITH READ ONLY option to your view definition. If you attempt to perform a DML on any rows in the view, you will see the Oracle Server error. SQL> CREATE OR REPLACE VIEW empvu10 2 (id_number, employee, title) 2 (id_number, employee, title) 3 AS SELECTempno, ename, job 3 AS SELECTempno, ename, job 4 FROMemp 4 FROMemp 5 WHEREdeptno = 10 5 WHEREdeptno = 10 6 WITH READ ONLY; 6 WITH READ ONLY; View created. SQL> CREATE OR REPLACE VIEW empvu10 2 (id_number, employee, title) 2 (id_number, employee, title) 3 AS SELECTempno, ename, job 3 AS SELECTempno, ename, job 4 FROMemp 4 FROMemp 5 WHEREdeptno = 10 5 WHEREdeptno = 10 6 WITH READ ONLY; 6 WITH READ ONLY; View created.

14-17 Database Systems Confirming Views The USER_VIEWS data dictionary table contains the name of the view and the view definition. SQL> SELECTview_name, text 2 FROMuser_views; 2 FROMuser_views; SQL> SELECTview_name, text 2 FROMuser_views; 2 FROMuser_views;

14-18 Database Systems Removing a View: Example Remove a view without losing data because a view is based on underlying tables in the database. SQL> DROP VIEW empvu10; View dropped. SQL> DROP VIEW empvu10; View dropped.

14-19 Database Systems Summary A view is derived from data in other tables or other views. A view is like a window to the underlying data. A view provides the following advantages: – Restrict database access – Simplify queries – Provide data independence – Allow multiple views of the same data – Can be dropped without removing the underlying data

14-20 Database Systems Practice Overview Creating a simple view Creating a complex view Creating a view with a check constraint Attempting to modify data in the view Displaying view definitions Removing views

14-21 Database Systems Practice 1 Create a view called DEPT20 that contains the employee number, name and department number for all employees in department 20 from EMP table. Label the view column EMPLOYEE_ID, EMPLOYEE and DEPARTMENT_ID. Do not allow an employee to be reassigned to another department through the view. Display the structure and contents of the DEPT20 view. Select the view name and text from the data dictionary USER_VIEWS. Attempt to reassign Smith to department 30.

14-22 Database Systems Practice 2 Create a view called SALARY_VU based on the employee name, depart name, salary and salary grade for all employees, using tables EMP, DEPT and SALGRADE. Label the columns Employee, Department, Salary and Grade respectively.