程序代写代做代考 database SQL Data Warehousing and Decision Support
Data Warehousing and Decision Support
Views
CS430/630
Lecture 11
Slides based on “Database Management Systems” 3rd ed, Ramakrishnan and Gehrke
Views
So far, we have looked at SQL tables
Relations that are persistent
Physically stored in the DBMS
It is also possible to have virtual relations, or views
Defined by an expression which is a SQL query
Do not exist physically in DBMS
Although it is possible to used materialized views
Views can be queried directly
In some cases, it is also possible to modify views
2
External
Schema
Levels of Abstraction
Data
Physical Schema
Conceptual Schema
View 1 View 2 View 3
Describes
files and
indexes
used
Defines
logical data
structure
Views
define how
users see
data
3
Creating a view
CREATE VIEW RegionalSales (category, sales, state)
AS SELECT P.category, S.sales, L.state
FROM Products P, Sales S, Locations L
WHERE P.pid=S.pid AND S.locid=L.locid
View
Defining Query
(also referred to as
View Subquery)
Base Tables
4
Querying views
SELECT R.category, R.state, SUM(R.sales)
FROM RegionalSales R GROUP BY R.category, R.state
Querying Views
Views are queried just like regular tables
A view is just another relation (albeit a virtual one)
Queries can involve both views and base tables
Helps to think of views in terms of analogy with window on data
5
Views as subqueries
SELECT R.category, R.state, SUM(R.sales)
FROM (SELECT P.category, S.sales, L.state
FROM Products P, Sales S, Locations L
WHERE P.pid=S.pid AND S.locid=L.locid) R
GROUP BY R.category, R.state
Equivalent Query
(without views)
SubQuery
6
Why are views useful?(1/3)
Usability
Certain information must be retrieved from many tables
View abstraction can get all info in one (virtual) table
Queries are much easier to write on a single table
Subqueries that are often used can be included in queries
without need for nesting
7
Why are views useful? (2/3)
Compatibility
Shield users and application developer from changes
What if a schema changes? Define view that looks like the old
schema
Users/applications access view, no changes needed in queries
“Obsolete” tables are preserved using views
8
Why are views useful? (3/3)
Security
Restrict user access to certain data only
Managers and employees are given different “views” of same data
Both column- and row-level access control possible
Column-wise: students can only access Name and Age
columns from a Student table
Row-wise: access only transactions above $10,000 value
9
Modifying views
Is it possible to insert, update, delete tuples in a view?
Views are virtual …
… so modifications must be reflected in the base tables
Why modifying views is a subtle issue?
Difficulty of translating view modifications in a unique way of
updating base tables
Must be non-ambiguous in how to trace the base table tuple to update
Views can be modified subject to restrictions
These are called updatable views
Still, many views are not updatable
10
Updatable Views
SQL-92 provides formal definition of updatable view:
1. View involves a single relation R. If R is a view, it must also be
updatable (relaxed in SQL-99)
2. Aggregate operations are not present in the view definition
3. The DISTINCT keyword is not specified in SELECT clause
4. All columns in subquery are simple columns, not expressions
5. The WHERE clause must not contain a subquery involving R
6. All attributes in R that are not in the SELECT clause of the
view must not have both NOT NULL restriction and no
default
11
Updatable Views (contd.)
Insertion can be done directly on the base table
Other attributes in R set to NULL
Deletion also possible
Delete tuple from base table
Both insertion and deletion may cause problems!
12
Issues with insertion
View Definition
Now let’s insert a new student
CREATE VIEW TopStudents (sname)
AS SELECT Name
FROM Students S
WHERE S.gpa > 3.0;
INSERT INTO TopStudents VALUES (‘FirstLastName’);
GPA is set to NULL
Tuple falls outside view definition!
Not a mistake, but update will not be reflected in view!
WITH CHECK OPTION clause disallows such an insertion
One solution is to include GPA in view definition
13
Issues with deletion
Now let’s delete students named Johnson
CREATE VIEW TopStudents (sname)
AS SELECT Name
FROM Students S
WHERE S.gpa > 3.0;
DELETE FROM TopStudents WHERE Name LIKE ‘%Johnson%’;
Must only affect tuples in the view!
Outside tuples must be inaccessible (views used for security, too)
DBMS appends WHERE clause in view definition to statement
DELETE FROM Students WHERE Name LIKE ‘%Johnson%’
AND S.gpa > 3.0;
14
Deleting views
View deleted from the schema
Note that, underlying data still intact
Contrast this with DROP TABLE!
DROP VIEW RegionalSales;
15
View Materialization
Materialized views can help speed up popular queries
Result has to be maintained when base tables change
They are stored just like base tables
But their contents are not “independent”; they must constantly
reflect base tables
16
Example
Create view ActorSummary that lists for every actor the actor identifier, actor
name, number of movies starred in, and the year of debut (i.e., the year of the
earliest movie(s) the actor starred in). The view will have four columns with
headings:
ID, ActorName, MovieCount and DebutYear
CREATE VIEW ActorSummary(ID, ActorName, MovieCount, DebutYear )
AS
SELECT A.actor_id, A.name, COUNT(M.movie_id), MIN(M.year)
FROM Actors A, StarsIn S, Movies M
WHERE A.actor_id = S.actor_id AND S.movie_id = M.movie_id
GROUP BY A.actor_id, A.name;
Example 2
CREATE VIEW ManagerSummary(DeptName, MgrName, MgrID,
MgrSalary, EmpCount) AS
SELECT D.dname, D.managerid, E.ename, E.salary, COUNT(W.eid)
FROM Department D, Employee E, Works W
WHERE D.managerid = E.eid AND D.did = W.did
GROUP BY D.did, D.dname, D.managerid, E.ename, E.salary;
Create a view ManagerSummary that lists for every department the
department name, manager ID and manager name, manager salary and
the number of employees in that department. The view will have five
columns with headings:
DeptName, MgrID, MgrName, MgrSalary and EmpCount.
