Chapter 11 – View Functions
"Be the change that you want to see in the world."
- Mahatma Gandhi
The Fundamentals of Views
View Fundamentals
A view is a virtual table.
A view may define a subset of columns.
A view can even define a subset of rows if it has a WHERE clause.
A view never duplicates data or stores the data separately.
Views provide security.
View Advantages
An additional level of security is provided.
Helps the business user not miss join conditions.
Help control read and update privileges.
Unaffected when new columns are added to a table.
Unaffected when a column is dropped unless its referenced in the view.
The above is designed to introduce View fundamentals, View advantages and View recommendations.
Creating a Simple View to Restrict Sensitive Columns
CREATE View Employee_V1 AS
SELECTEmployee_No
,First_Name
,Last_Name
,Dept_No
FROM Employee_Table ;
The purposes of views is to restrict access to certain columns, derive columns or Join Tables, and to restrict access to certain rows (if a WHERE clause is used). This view does not allow the user to see the column salary.
You SELECT From a View
You SELECT from a view exactly like you SELECT from a table. A user might not know whether or not they were querying a view or a table.
Creating a Simple View to Restrict Rows
The purposes of views is to restrict access to certain columns, derive columns or Join Tables, and to restrict access to certain rows (if a WHERE clause is used). This view does not allow the user to see information about rows unless the rows have a Dept_No of either 300 or 400.
A View Provides Security for Columns and Rows
CREATE VIEW Employee_V2 AS
SELECT First_Name
,Last_Name
,Dept_No
,Salary
FROMEmployee_Table
WHEREDept_No IN (300, 400) ;
A view is usually associated with protecting sensitive columns, but view that utilize a WHERE clause also protect sensitive rows from being seen. Also, notice that we did not put an ORDER BY statement in the view creation, but instead you utilize the ORDER BY statement when you query the view.
Basic Rules for Views
No ORDER BY inside the View CREATE (exceptions exist)
All Aggregation needs to have an ALIAS
Any Derived columns (such as Math) needs an ALIAS
Above are the basic rules of Views with excellent examples.
How to Modify a View
The CREATE OR REPLACE Keywords will allow a user to change a view.
An Exception to the ORDER BY Rule inside a View
There are EXCEPTIONS to the ORDER BY rule when creating a view. ANSI OLAP statements that use the ORDER BY statement also work inside a View.
Views Are Sometimes CREATED for Formatting
CREATE VIEW Order_V5 AS
SELECT
Order_Number
,Customer_Number
, TO_CHAR(Order_Date , 'MON, DD, YYYY') AS Order_Date
,TO_CHAR(Order_Total, 'L999,999.99') AS Total
FROM Order_Table ;
SELECT *
FROM Order_V5 ;
Views are designed to do many things. In the example above, this view formats some of the data.
Creating a View to Join Tables Together
This view is designed to join two tables together. By creating a view, we have now made it easier for the user community to join these tables by selecting the columns you want from the view or even the rows.
How to Alias Columns in a View CREATE
SELECT *
FROM E_View
ORDER BY Mnth_Sal ;
Columns in a view that are derived, such as aggregates or calculated columns, must have an alias. There are multiple ways to alias the columns and this example shows you the first option.
The Standard Way Most Aliasing is Done
SELECT *
FROM E_View2
ORDER BY Sal_Monthly ;
The ALIAS for Salary / 12 that’ll be used in this example is Sal_Monthly and this form of aliasing is most often used.
What Happens When Both Aliasing Options Are Present
SELECT *
FROM E_View3
ORDER BY 3 ;
The ALIAS for Salary / 12 that’ll be used in this example is Mnth_Sal. It came first at the top, even though it is aliased in the SELECT list also.
Resolving Aliasing Problems in a View CREATE
What happens when this query runs?
What will happen in the above query?
Answer to Resolving Aliasing Problems in a View CREATE
What happens when this query runs?
Error – Sal_Mnth is unrecognized
The query above errors because Sal_Mnth is an unrecognized alias. That is because we did our aliasing at the top so this makes the alias right after Salary/12 non-valid for use when querying the view.
Aggregates on View Aggregates
CREATE VIEW Aggreg_Order_v AS
SELECT
Customer_Number
,COUNT(Order_Total) AS Order_Cnt
,SUM(Order_Total)AS Order_Sum
,AVG(Order_Total)AS Order_Avg
FROMOrder_Table
GROUP BY Customer_Number ;
The examples above show how we put a SUM on the aggregate Order_Sum!
Altering A Table After a View Has Been Created
Altering the actual Table
This view will run after the table has added an additional column!
A View that Errors After An ALTER
Altering the actual Table
ALTER TABLE Employee_Table4
Drop COLUMN Dept_No ;
SELECT * FROM Emp_HR_V4
ERROR [HY000] [Nexus Chameleon][ODBC Oracle Wire Protocol
driver][Oracle]ORA-04063: view "SQL_CLASS.EMP_HR_V5" has
errors SELECT Command Failed.
This table will error if you try to drop a column that exists in a view.