Lecture 15 Section 5.3 Robb T. Koetherpeople.hsc.edu/faculty-staff/robbk/Coms480/Lectures...Views Lecture 15 Section 5.3 Robb T. Koether Hampden-Sydney College Mon, Feb 18, 2013 Robb

ViewsLecture 15Section 5.3

Robb T. Koether

Hampden-Sydney College

Mon, Feb 18, 2013

Robb T. Koether (Hampden-Sydney College) Views Mon, Feb 18, 2013 1 / 27

1 Views

2 Modifying the Base Tables

3 Updating Views

4 Using Triggers to Update Views

5 Assignment


Outline

1 Views


3 Updating Views


5 Assignment


Views

A view is a virtual table constructed from physical tables.A view can be used as a table (select, insert, delete, update), butit does not exist as a physical table in the database.A query on the view must be interpreted as a query on the basetables from which the query is constructed.At times, this can lead to significant complications.


Views

Views are useful forConstructed tables, such as joins, that are used frequently.For security, to hide information that a user is not authorized to see.


Creating a View

Creating a ViewCREATE VIEW view_name AS select_statement;

The view is defined to be those tuples that are returned by theSELECT statement.


Creating a View

Creating a ViewCREATE VIEW emp_w_dep ASSELECT fname, lname, ssn, salary, bdateFROM employeesWHERE ssn IN(SELECT ssn FROM dependents)

For example, we might want to create a view of those employeeswho have dependents.


Creating a View

Creating a ViewSELECT * FROM emp_w_dep;+----------+---------+-----------+-----------+------------+| fname | lname | ssn | salary | bdate |+----------+---------+-----------+-----------+------------+| James | Green | 246813579 | 100000.00 | 1974-02-15 || Jennifer | Wallace | 321549876 | 50000.00 | 1985-12-02 || Frank | Gilbert | 369147258 | 30000.00 | 1966-08-21 || Joyce | English | 456789012 | 25000.00 | 1983-05-07 || Richard | Johnson | 531978642 | 35000.00 | 1955-03-17 || John | Kohler | 789012345 | 40000.00 | 1966-11-24 || Raymond | Jones | 963418527 | 80000.00 | 1974-08-30 |+----------+---------+-----------+-----------+------------+


Views as Tables

Views as TablesSHOW TABLES;+-------------------+| Tables_in_company |+-------------------+| departments || dependents || employees || projects || emp_w_dep || works |+-------------------+

Views are included in the list of tables.We can use SHOW CREATE VIEW to display he details of a view.


Selecting From a View

Selecting From a ViewSELECT fname, lnameFROM emp_w_depWHERE salary > 50000;+---------+-------+| fname | lname |+---------+-------+| James | Green || Raymond | Jones |+---------+-------+

There is never a problem with selecting from a view.



Selecting From a ViewSELECT fname, lname, dep_nameFROM emp_w_dep AS e, dependents AS dWHERE e.ssn = d.ssn;+----------+---------+----------+| fname | lname | dep_name |+----------+---------+----------+| James | Green | Debbie || James | Green | Sarah || Jennifer | Wallace | Fred || Jennifer | Wallace | Jimmy || Jennifer | Wallace | Susie || Frank | Gilbert | Laura || Frank | Gilbert | Rachel || Joyce | English | Bobby || Richard | Johnson | Patty || John | Kohler | Sharon || Raymond | Jones | Donna |+----------+---------+----------+

There is never a problem with selecting from a view.


Outline

1 Views


3 Updating Views


5 Assignment



Modifying the Base TablesUPDATE employeesSET fname = ’Michael’WHERE fname = ’Frank’;

SELECT * FROM emp_w_dep;+----------+---------+-----------+-----------+------------+| fname | lname | ssn | salary | bdate |+----------+---------+-----------+-----------+------------+| James | Green | 246813579 | 100000.00 | 1974-02-15 || Jennifer | Wallace | 321549876 | 50000.00 | 1985-12-02 || Michael | Gilbert | 369147258 | 30000.00 | 1966-08-21 || Joyce | English | 456789012 | 25000.00 | 1983-05-07 || Richard | Johnson | 531978642 | 35000.00 | 1955-03-17 || John | Kohler | 789012345 | 40000.00 | 1966-11-24 || Raymond | Jones | 963418527 | 80000.00 | 1974-08-30 |+----------+---------+-----------+-----------+------------+

If the base tables are changed, then the view will changeautomatically.


Outline

1 Views


3 Updating Views


5 Assignment


Updating Views

What happens to the base tables if we modify the view?In the previous example,

What happens if we insert the tuple(’Brian’, ’Jacobson’, ’888776666’, 40000.00, ’1990-04-19’)

into emp_w_dep?What happens if we update that tuple?What happens if we delete that tuple?


Updating Views

Updating a ViewINSERT INTO emp_w_depVALUES(’Brian’, ’Jacobson’, ’888776666’, 40000.00, ’1990-04-19’);Query OK, 1 row affected, 1 warning (0.00 sec)

SHOW WARNINGS;+---------+------+---------------------------------------------------------------------------------+| Level | Code | Message |+---------+------+---------------------------------------------------------------------------------+| Warning | 1423 | Field of view ’company.emp_w_dep’ underlying table doesn’t have a default value |+---------+------+---------------------------------------------------------------------------------+

SELECT * FROM employees WHERE ssn = ’888776666’;+-------+----------+-----------+------------+------+----------+------+| fname | lname | ssn | bdate | sex | salary | dept |+-------+----------+-----------+------------+------+----------+------+| Brian | Jacobson | 888776666 | 1990-04-19 | NULL | 40000.00 | 0 |+-------+----------+-----------+------------+------+----------+------+

Also, update and delete the tuple.In general insertions, deletions, and updates are permittedprovided they affect only one base table.


Updating Views

The emp_w_dep view is based on only one table (or was it?).What would happen if we added or deleted dependents?


Updating Views

Inserting Into a Base TableINSERT INTO dependentsVALUES(’123456789’, ’Tiffany’, ’F’, ’2013-02-18’);

SELECT * FROM emp_w_dep;+----------+---------+-----------+-----------+------------+| fname | lname | ssn | salary | bdate |+----------+---------+-----------+-----------+------------+| Alice | Smith | 123456789 | 35000.00 | 1968-05-22 || James | Green | 246813579 | 100000.00 | 1974-02-15 || Jennifer | Wallace | 321549876 | 50000.00 | 1985-12-02 || Michael | Gilbert | 369147258 | 30000.00 | 1966-08-21 || Joyce | English | 456789012 | 25000.00 | 1983-05-07 || Richard | Johnson | 531978642 | 35000.00 | 1955-03-17 || John | Kohler | 789012345 | 40000.00 | 1966-11-24 || Raymond | Jones | 963418527 | 80000.00 | 1974-08-30 |+----------+---------+-----------+-----------+------------+


Updating Views

Deleting From a Base TableDELETE FROM dependentsWHERE ssn = ’123456789’;

SELECT * FROM emp_w_dep;+----------+---------+-----------+-----------+------------+| fname | lname | ssn | salary | bdate |+----------+---------+-----------+-----------+------------+| James | Green | 246813579 | 100000.00 | 1974-02-15 || Jennifer | Wallace | 321549876 | 50000.00 | 1985-12-02 || Michael | Gilbert | 369147258 | 30000.00 | 1966-08-21 || Joyce | English | 456789012 | 25000.00 | 1983-05-07 || Richard | Johnson | 531978642 | 35000.00 | 1955-03-17 || John | Kohler | 789012345 | 40000.00 | 1966-11-24 || Raymond | Jones | 963418527 | 80000.00 | 1974-08-30 |+----------+---------+-----------+-----------+------------+


Views and Joins

Updating views is more difficult when the view is based on morethan one table.


Views and Joins

Creating a ViewCREATE VIEW emp_hrs ASSELECT fname, lname, pname, hoursFROM employeesNATURAL JOIN projectsNATURAL JOIN works;

This view is based on the join of three tables.

Uh-oh.


Views and Joins

Creating a ViewCREATE VIEW emp_hrs ASSELECT fname, lname, pname, hoursFROM employeesNATURAL JOIN projectsNATURAL JOIN works;

This view is based on the join of three tables.Uh-oh.


Views and Joins

Creating a ViewSELECT * FROM emp_hrs;+----------+---------+-----------------+-------+| fname | lname | pname | hours |+----------+---------+-----------------+-------+| Alice | Smith | Payroll | 40.0 || John | Kohler | Payroll | 40.0 || Raymond | Jones | Payroll | 20.0 || Jennifer | Wallace | Investment | 40.0 || Ernest | Roth | Investment | 40.0 || Raymond | Jones | Investment | 20.0 || Barbara | Brown | Clothing | 20.0 || Michael | Gilbert | Clothing | 40.0 || Joyce | English | Clothing | 20.0 || Amy | Ford | Clothing | 30.0 || Barbara | Brown | Office Supplies | 10.0 || Joyce | English | Office Supplies | 20.0 || Amy | Ford | Office Supplies | 10.0 || Barbara | Brown | Housewares | 10.0 || Richard | Johnson | Housewares | 40.0 |+----------+---------+-----------------+-------+


Inserting Into a View

Inserting Into a ViewINSERT INTO emp_hrs VALUES(’Jason’, ’Simpson’, ’Clothing’, 25.0);ERROR 1394 (HY000): Can not insert into join view ’company.emp_hrs’without fields list

INSERT INTO emp_hrs(fname, lname, pname, hours)VALUES(’Jason’, ’Simpson’, ’Clothing’, 25.0);ERROR 1393 (HY000): Can not modify more than one base table througha join view ’company.emp_hrs’

Try to insert into the view.Try updating or deleting.


Outline

1 Views


3 Updating Views


5 Assignment


Inserting Into a View

Inserting Into a ViewCREATE TRIGGER add_emp_depAFTER INSERT ON emp_w_depFOR EACH ROWINSERT INTO dependentsVALUES(NEW.ssn, ’’, null, null);

ERROR 1347 (HY000): ’company.emp_w_dep’ is not BASE TABLE

Normally, modifying a view would have been handled by triggersthat modify the base tables in the appropriate way.Unfortunately, MySQL does not support triggers on views.


Outline

1 Views


3 Updating Views


5 Assignment


Assignment

AssignmentRead Section 5.3.


Documents

Lecture 15 Section 5.3 Robb T. Koetherpeople.hsc.edu/faculty-staff/robbk/Coms480/Lectures...Views Lecture 15 Section 5.3 Robb T. Koether Hampden-Sydney College Mon, Feb 18, 2013 Robb