SQL – Ch 13 – SQL VIEWS

Prof. Mukesh N. Tekwani [9869 488 356] Page 1

13. SQL - VIEWS

1 What is a view?

1. A view is a derived table. A view does not store any rows itself. It can combine data from many tables to

make a ‘virtual’ table. The actual table on which a view is based is called the ‘base table’.

2. Views are useful for presenting data in different ways to different users. This saves typing in the common

queries.

3. A view is not a snapshot of the data returned by a particular query. A view is a copy of the SELECT

statement itself.

4. Every time we access a view. The query on which the view is based is run, and the data currently in the

base tables is returned.

2 What are the advantages of a view?

1. Security: Each user can be given permission to access the database only through a set of views. This

restricts the user's access to stored data.

2. Convenience: A view can take data from many tables and show it as a single table; this converts multi-

table queries into single-table queries.

3. Structural simplicity: The user can have a "personalized" view of the database. 4. No changes in output as seen by user: A view can present an unchanged image of the database,

even if the source tables are split, restructured, or renamed.

5. Data integrity: If data is accessed and entered through a view, the DBMS can automatically check the

data to ensure that it meets specified integrity constraints.

6. Restricting data available to users: If we want to provide access to some small part of data store in a

table, a view is useful.

3 What are the disadvantages of a view?

1. Performance: If a view is defined by a complex, multi-table query, then even a simple query against the

view becomes a complicated join, and it may take a long time to complete. 2. Update restrictions: When a user tries to update rows of a view, the DBMS must translate the request

into an update on rows of the underlying source tables. This is possible for simple views, but more

complex views cannot be updated.

4 Creating a View

The CREATE VIEW statement is used to create a view.

Syntax: CREATE VIEW viewname

• Assign a name to each column in the view. If a list of column names is specified, it must have the same number of items as the number of columns in the query.

• Only the column names are specified; we should not give the other characteristics of columns.

• If the list of column names is omitted from the CREATE VIEW statement, each column in the

view takes the name of the corresponding column in the query.

• The list of column names must be specified if the query includes calculated columns or if it produces two columns with identical names.

Horizontal View:

Example: CREATE VIEW PASSED AS

SELECT * FROM STUDENTS

WHERE RESULT = ‘P’

A horizontal view will restrict the user’s access to only a few rows of the table. A “horizontal view” will permit

a user to see only the records of those who have passed. A horizontal view slices the table horizontally to create

the view. All columns are included in the view but only some of the rows are displayed.

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Example 2:

Define a view for Sue (employee number 102) containing only orders placed by customers assigned to her. CREATE VIEW SUEORDERS AS SELECT *

FROM ORDERS WHERE CUST IN

(SELECT CUST_NUM FROM CUSTOMERS WHERE CUST_REP = 102)

Vertical View: A vertical view restricts a user’s access to only certain columns of a table.

Example 1: CREATE VIEW STUD_ADDRESS AS

SELECT ROLLNO, NAME, ADD1, ADD2, CITY

FROM STUDENTS

Row/Column Subset View:

This type of view is a combination of horizontal and vertical views. It will display a few selected rows

satisfying the condition in the WHERE clause, and a few selected columns.

Example: CREATE VIEW STUD_PASSED AS

SELECT ROLLNO, NAME, PCTG

FROM STUDENTS

WHERE RESULT = ‘P’

Grouped View:

A grouped view is one in which a query includes the GROUP BY clause. Related rows of data are grouped

together and produce one row of result for each group.

Define a view that contains summary order data for each salesperson. CREATE VIEW ORD_BY_REP (WHO, HOW_MANY, TOTAL, LOW, HIGH, AVERAGE) AS

SELECT REP, COUNT(*), SUM(AMOUNT), MIN(AMOUNT), MAX(AMOUNT), AVG(AMOUNT)

FROM ORDERS

GROUP BY REP

5 Examples of CREATE VIEW

a) Create a view to print names of all movies in capital letters.

CREATE VIEW Movies_upper(title)

AS

SELECT UPPER(movie_title)

FROM Movies

b) Create a view to find the total revenue from all movies.

CREATE VIEW TRev (total_revenue)

AS SELECT SUM(gross)

FROM Movies

c) Create a view to find all people who live in a state where a movie studio is located. CREATE VIEW PS AS

SELECT FName, LName, StudioName, Person_State FROM People, Studio

WHERE Person_State = Studio_State

SQL – Ch 13 – SQL VIEWS

Prof. Mukesh N. Tekwani [9869 488 356] Page 3

d) This is an example of a view with a sub-query. Create a view that returns the list of all the studios with an average budget of over $50 million. CREATE VIEW Big_Movies AS

SELECT movie_title, budget, gross FROM Movies

WHERE studio_id IN

(SELECT studio_id

FROM Movies

GROUP BY studio_id

HAVING AVG(budget) > 50)

6 Updating a View

Records can be updated, inserted, and deleted though views. Views against which INSERT, DELETE, and UPDATE statements can be used are called as updateable views. The following conditions must be fulfilled for view updates: 1. Each row in the view must map to a single row in the base table.that are used in the view. 2. Aggregate functions, the DISTINCT operator, GROUP BY clause and HAVING clause cannot be

used in updateable views. 3. The WHERE clause cannot include a sub-query. 4. The VIEW must have a single source table 5. Calculated columns cannot be use in view updates. 6. Updating a view is actually updating the underlying (base) table – i.e. the table mentioned in the

SELECT clause. 7. But it may happen that even though the base table is updated, the new record does not show up

in the view due to the nature of the view.

7 Explain the CHECK OPTION for VIEW updates.

If a view is defined by a query that includes a WHERE clause, only rows that meet the search

condition are visible in the view. Other rows may be present in the source table(s) from which the view is derived, but they are not visible through the view. For example, consider the EASTREPS view contains which only few rows of the SALESREPS table:

CREATE VIEW EASTREPS AS SELECT *

FROM SALESREPS WHERE REP_OFFICE IN (11, 12, 13)

We can add a new record with this updateable view:

INSERT INTO EASTREPS (EMPL_NUM, NAME, REP_OFFICE, AGE, SALES) VALUES (113, 'Jake', 11, 43, 0.00)

The DBMS will add the new row to the underlying SALESREPS table, and the row will be visible through the EASTREPS view. The row we are trying to insert satisfies the condition that the

REP_OFFICE should be 11, 12, or 13. But suppose we add a new salesperson with this INSERT statement:

INSERT INTO EASTREPS (EMPL_NUM, NAME, REP_OFFICE, AGE, SALES) VALUES (114, 'Fred’, 21, 47, 0.00)

Here the REP_OFFICE is 21. The row in inserted into the underlying table, but it is not displayed by the view.

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This can cause problems with data integrity. To avoid such problems (table updated but record not displayed by view), SQL provides the CHECK OPTION as follows: CREATE VIEW EASTREPS AS

SELECT *

FROM SALESREPS

WHERE REP_OFFICE IN (11, 12, 13)

WITH CHECK OPTION

Now, if we try to use an update view, SQL will automatically check each UPDATE and INSERT query to make sure that the resultant query meets the condition “REP_OFFICE IN (11, 12, 13)”.

CASCADED and LOCAL options When a new view is created it may be based on another view, and not on a table. E.g., a view VC may be based

on view VB, which in turn is based on view VA. We say that there is a hierarchy of views with VA being at

the highest level, than VB and finally VC.

If the new view VC is created WITH CASCADED CHECK OPTION, and we attempt to update the

view, it causes the DBMS go to view VB and then VA to check option for each view. If the new view is created WITH LOCAL CHECK OPTION, then the DBMS checks only that view; the underlying views are not checked. So if we define view VC with LOCAL CHECK OPTION, then only

conditions specified in that view are checked.

8 Dropping a View – CASCADE and RESTRICT options

When a view is no longer needed, it can be removed by using the DROP VIEW statement. We can also control what happens when a view is dropped and another view depends on its definition. E.g., consider that we have view VA and VB. View VB depends on VA. We give the query DROP VIEW VA

If we drop VA, then cascading effect takes place and view VB is also dropped. Thus the default option for dropping a view is CASCADE. The CASCADE option tells the DBMS to delete not only the

named view, but also any views that depend on its definition. But if we give the query DROP VIEW VA RESTRICT

Now the query fails because RESTRICT option tells the DBMS to remove the view only if no other

views depend on it. Since VB depends on VA, it will cause an error.