The SQL Query Language DML1 The SQL Query Language DML Odds and Ends

The SQL Query Language DML 1

The SQL Query Language DML

The SQL Query LanguageDML Odds and Ends


Outline

Data creation and destruction Inserting into a table Deleting from a table Modifying values in a table

Other commonly used features Views Transactions and triggers

Summary


Modifications

There are three modification statements INSERT UPDATE DELETE

For insertions, either values can be specified, or a select statement provides the values

Enter a reservation for Eric for the film 332244

INSERT INTO Reserved

VALUES (123456, 332244, CURRENT_DATE)


INSERT, cont.

Let Melanie reserve all the films that Eric has reserved.

INSERT INTO Reserved

SELECT C2.CustomerID, FilmID, CURRENT_DATE

FROM Reserved, Customer C, Customer C2

WHERE C.Name = ’Eric’AND C.CustomerID = Reserved.CustomerID AND C2.Name = ’Melanie’


DELETE

A where clause identifies which rows to remove from the table

Delete all the reservations of customer 123456

DELETE FROM Reserved

WHERE CustomerID = 123456

Other tables can be consulted to determine which rows should be removed

Delete all of Eric's reservations

DELETE FROM ReservedWHERE CustomerID = ANY(SELECT CustomerID

FROM Customer WHERE Name = ’Eric’)


DELETE, cont

Delete the films with a rental price below the average

DELETE FROM Film WHERE RentalPrice <(SELECT AVG(RentalPrice) FROM Film)

Implementation difficulty: As we delete rows from Film, the average rental balance changes Not a problem

Inner query, first computes the average balanceOuter “loop” deletes tuples without recomputing AVG


UPDATE

Increase the rental price of all films by 10%UPDATE Film

SET RentalPrice = RentalPrice * 1.10

The update statement has an optional where clause Increase the rental price of foreign films by 10%

UPDATE FilmSET RentalPrice = RentalPrice * 1.10WHERE Kind = ’F’


UPDATE, cont

Increase the rental price of films with a current price of under $3 by 20%, and all others by 10%

UPDATE FilmSET RentalPrice = RentalPrice * 1.10WHERE RentalPrice >= 3.00

UPDATE FilmSET RentalPrice = RentalPrice * 1.20WHERE RentalPrice < 3.00

Careful, order is important!


Outline

Data creation and destruction Inserting into a table Deleting from a table Modifying values in a table

Other commonly used features Views Transactions and triggers

Summary


View

Views provide a mechanism to create a virtual tableCREATE VIEW name AS query expression

To create a view we use the command Define a view of all customers in Dublin

CREATE VIEW Dublin_Customers ASSELECT *

FROM CustomerWHERE City = ’Dublin’


View, cont.

Define a view of all customers holding reservations, and the films they have reserved

CREATE VIEW Reservations ASSELECT Name, TitleFROM Customer, Reserved, FilmWHERE Customer.CustomerID = Reserved.CustomerID

AND Reserved.FilmID = Film.FilmID


Transactions

A transaction identifies an elementary unit of work carried out by an application, to which we wish to allocate particular characteristics of reliability and isolation

A system that makes available mechanisms for the definition and execution of transactions is called a transaction processing system

Transactions are initiated with any SQL statement that modifies the database


Transactions

A transaction can be defined syntactically: each transaction, irrespective of the language in which it is written, is enclosed whthin two commands

begin transaction

end transaction Within the transaction code, two particular

instructions can appear

commit work

rollback work


Transaction example

An example of transaction is given in the following code

begin transaction

X := x – 10;

Y := y + 10;

Commit work;

end transaction


Active databases

An active database system is a DBMS that supports an integrated subsystem for the definition and management of production rules

The rules follow the event – condition – action paradigm: each rule reacts to some events, evaluates a condition and, based on the truth value of the condition, might carry out an action

The execution of the rules happens under the control of an autonomous subsystem, known as the rule engine, which keeps track of the events that have occurred and schedules the rules for execution

One of the active rules called triggers


Triggers

The creation of triggers is part of the DDL Maintain data integrity Associated with a table (view) Event-condition-action

Wait for a table event

On event, evaluate condition If condition is true, execute action

Xbefore after

insertion deletion update


How to create a trigger (SQL Server Enterprise Manager)

1. Expand a server group, and then expand a server.

2. Expand Databases, expand the database in which the table to contain the trigger belongs, and then click Tables.

3. In the details pane, right-click the table on which the trigger will be created, point to All Tasks, and then click Manage Triggers.

4. In Name, click <new>.

5. In the Text box, enter the text of the trigger. Use CTRL-TAB to indent the text of a trigger.

6. To check the syntax, click Check Syntax.


Syntax (SQL server)

CREATE TRIGGER trigger_name ON { table | view } [ WITH ENCRYPTION ] { { { FOR | AFTER | INSTEAD OF } { [ INSERT ] [ , ] [ UPDATE ] } [ WITH APPEND ] [ NOT FOR REPLICATION ] AS [ { IF UPDATE ( column ) [ { AND | OR } UPDATE ( column ) ] [ ...n ] | IF ( COLUMNS_UPDATED ( ) { bitwise_operator } updated_bitmask ) { comparison_operator } column_bitmask [ ...n ] } ] sql_statement [ ...n ] } }


Oracle Table-level Trigger Body

Example: At most 100 people can live in Dublin After update/insert into Person

DECLARE Declare Variables

C INTEGER;

BEGIN Body of Trigger

SELECT COUNT(Name) INTO C

FROM Person

WHERE City = ‘Dublin’;

IF (C > 100) THEN

RAISE_APPLICATION_ERROR(-20000, ‘too many in Dublin’);

END IF;

END


Potential Applications

Notification an active database may be used to monitor

Enforce integrity constraints Business roles

Maintenance of derived data Maintain the derived attribute whenever individual tuples are

changed


Trigger Gotchas

Potentially infinite loop Trigger A: On insertion into Person, insert into Population Trigger B: On insertion into Population, insert into Person

Mutating tables Trigger A: On insertion into Person, insert into Person! Disallowed! Trigger cannot make changes to table that trigger is defined

on


Summary

Schema Definition CREATE TABLE CREATE VIEW ALTER DROP

Queries SELECT

Modifications INSERT DELETE UPDATE

Transaction Management COMMIT ROLLBACK

Active Database Trigger

Documents

The SQL Query Language DML1 The SQL Query Language DML Odds and Ends