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TriggersTriggers

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PL/SQL reminderPL/SQL reminder

• We presented PL/SQL- a Procedural

extension to the SQL language.

• We reviewed the structure of an anonymous

PL/SQL block:

DECLARE (optional) /* Variable declaration */

BEGIN (mandatory) /* Block action*/

EXCEPTION (optional) /* Exception handling */

END; (optional)/

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Example from last weekExample from last weekDECLARE

e_number1 EXCEPTION;

cnt NUMBER;

BEGIN

select count(*)

into cnt

from number_table;

IF cnt = 1 THEN RAISE e_number1;

ELSE dbms_output.put_line(cnt);

END IF;

EXCEPTION

WHEN e_number1 THEN

dbms_output.put_line('Count = 1');

end;

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PL/SQL reminder-cont.PL/SQL reminder-cont.• We also showed the structures of procedures

and functions:

create or replace procedure num_logged(person IN mylog.who%TYPE, num OUT mylog.logon_num%TYPE)ISBEGIN select logon_num into num from mylog where who = person;END;/

create or replace procedure num_logged(person IN mylog.who%TYPE, num OUT mylog.logon_num%TYPE)ISBEGIN select logon_num into num from mylog where who = person;END;/

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Triggers- introductionTriggers- introduction

• A trigger is an action which the Database

should perform when some DB event has

occurred.

• For example (in pseudocode):

TriggerA:

For any row that is inserted into table Sailors:

if age>30 -> insert this row into oldSailors;

else-> insert this row into youngSailors;

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Trigger introduction cont.Trigger introduction cont.

• The code within the trigger, called the trigger body, is made up of PL/SQL blocks

• The “firing” of a trigger is transparent to the user.

• There are many optional triggering events, but we will focus on update, delete, and insert.

• Triggers can be used to check for data integrity, but should be used so only if it is not possible through other means.

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Types of triggersTypes of triggers

1. Row level triggers: The code in the trigger is executed once for every row updated.

2. Statement level triggers (Default): The code in the trigger is performed once per statement.

• For example: if the triggering event was an update which updates 100 rows, a row-level trigger will execute 100 times, and a statement level trigger will execute once.

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Types of triggers-cont.Types of triggers-cont.

1.BEFORE triggers: The trigger fires immediately BEFORE the triggering event executes.

2.AFTER triggers: The trigger fires immediately AFTER the triggering event executes.

3.INSTEAD OF triggers: The trigger fires INSTEAD of the triggering event.

• We can reference the “old” and “new” values.

• If we want to change rows which will be inserted, we have to use a BEFORE trigger and change the ‘new’ values. Using an AFTER trigger will not allow the change.

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Example (pseudocode)Example (pseudocode)

Create Before-Trigger:

– For every string inserted into sailorName,

turn it into upper case before insertion

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Trigger syntaxTrigger syntax

CREATE [or REPLACE] TRIGGER trig_name{BEFORE | AFTER | INSTEAD OF}{DELETE | INSERT | UPDATE}

[of column1, column2,…] [or {DELETE | INSERT | UPDATE]

[of columnA, columnB,…]

on table_name

[FOR EACH ROW][WHEN (condition)]

PL/SQL block

Further restricts when trigger is

fired

Triggering event

Trigger timing

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Backing Up DataBacking Up Data

create table sailors_backup( who varchar2(30), when_chngd date, sid number, old_rating number, new_rating number);

create table sailors(sid number,sname VARCHAR2(30),rating number check(rating <= 10),age number

);

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sailors(sid,sname,rating,age);sailors(sid,sname,rating,age);sailors_backup(who,when_chngd,sid, sailors_backup(who,when_chngd,sid,

old_rating,new_rating);old_rating,new_rating);

• Q: Why AFTER Trigger?

• A: Because in that case, the firing of the trigger occurs only when the inserted data complies with the table integrity (check..)

CREATE or REPLACE TRIGGER backup_trigAFTER UPDATE of Rating on SailorsFOR EACH ROW WHEN (old.rating < new.rating)BEGIN

INSERT INTO sailors_backupVALUES (USER, SYSDATE, :old.sid,

:old.rating, :new.rating);END;/

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Ensuring Upper CaseEnsuring Upper Case

CREATE or REPLACE TRIGGER sname_trigBEFORE INSERT or UPDATE of snameon SailorsFOR EACH ROWBEGIN

:new.sname := UPPER(:new.sname);

END;/

• Q: Why BEFORE Trigger?

• A: You cannot update inserted values after insertion

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Instead Of TriggerInstead Of Triggercreate view sailors_reserves asselect sailors.sname, reserves.bidfrom sailors, reserveswhere sailors.sid = reserves.sid;

CREATE or REPLACE TRIGGER view_trigINSTEAD OF INSERT on sailors_reservesFOR EACH ROWBEGININSERT INTO sailors values(:new.sname);INSERT INTO reserves values(:new.bid);END;/

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Statement TriggerStatement TriggerCREATE or REPLACE TRIGGER no_work_on_shabbat_trigBEFORE INSERT or DELETE or UPDATEon reservesDECLARE

shabbat_exception EXCEPTION;

BEGINif (TO_CHAR(sysdate,'DY')='SAT') then

raise shabbat_exception;end if;

END;/

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Another exampleAnother examplecreate or replace trigger trig2

after update of rating on sailors

for each row

DECLARE

diff number:=abs((:old.rating)-(:new.rating));

BEGIN

If ((:old.rating)>(:new.rating)) then dbms_output.put_line('The rating of '||:old.sname||' has dropped by '||diff);

elsif ((:old.rating)<(:new.rating)) then dbms_output.put_line('The rating of '||:old.sname||' has been raised by '||diff);

else dbms_output.put_line('The rating of '||:old.sname||' has remained the same');

end if;

END;

/

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Trigger Compilation ErrorsTrigger Compilation Errors

• As with procedures and functions, when creating a Trigger with errors, you will get the message:

“Warning: Trigger created with compilation errors.”

To view the errors, type:

SHOW ERRORS TRIGGER myTrigger

• To drop a trigger write

drop trigger myTrig

• To disable/enable a trigger write

alter trigger myTrig disable/enable

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Additional Types of Additional Types of TriggersTriggers

• Can also define triggers for

– logging in and off

– create/drop table events

– system errors

– etc.

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JDBCJDBCTMTM

"Java Database Connectivity""Java Database Connectivity"

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• Getting Started Guide:

http://java.sun.com/j2se/1.5.0/docs/guide/jdbc/

getstart/GettingStartedTOC.fm.html

• java.sql Package API:

http://java.sun.com/j2se/1.5.0/docs/api/java/sql/package-summary.html

Useful JDBC LinksUseful JDBC Links

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Introduction to JDBCIntroduction to JDBC

• JDBC allows for convenient database

access from Java applications

• Data is transferred from relations to

objects and vice-versa

– databases optimized for searching/indexing

– objects optimized for engineering/flexibility

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Why Access a Database from Why Access a Database from within a Program?within a Program?

• As we saw last week, some queries can’t be computed in SQL.

• PL/SQL includes more programming tools than SQL

• However, sometimes using PL/SQL will not be suitable:

– If we require object-oriented programming

– If accessing the database is a small part of a large Java application

– If we would like to access other DBMSes than Oracle

– Etc.

• Why not keep all the data in Java objects?

• “Separation of concerns”: DBMSes concentrate on data storage and access; programs concentrate on algorithms, networking, etc.

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Six StepsSix Steps

• Load the driver

• Establish the Connection

• Create a Statement object

• Execute a query

• Process the result

• Close the connection

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Packages to ImportPackages to Import

• In order to connect to the Oracle

database from java, import the

following packages:

– java.sql.*; (usually enough)

– javax.sql.* (for advanced features, such

as scrollable result sets)

JDBC Architecture (1)JDBC Architecture (1)

Application

• DriverManager is provided by Java

Software as part of the Java 2

Platform.

• Drivers are provided by DBMS

vendors.

Driver

DriverManager

DBMS

JDBC Architecture (2)JDBC Architecture (2)

• The application creates a driver instance and registers it with the DriverManager.

• The DriverManager tells the driver to connect to the DB

• The DriverManager keeps track of registered driver instances and their connections to DB’s.

• The Driver “talks” to a particular DB through the connection

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ConnectingConnecting

1. Initializing a driver: new oracle.jdbc.driver.OracleDriver()

2. Registering it with the DriverManager: DriverManager.registerDriver(…)

3. Getting a connection:

DriverManager.getConnection(URL)Note:

• Stages 1+2 may be combined with Class.forName(“oracle.jdbc.driver.OracleDriver");

• In Stage 3, the Manager tries all drivers and assigns a connection to the first driver that succeeds.

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Connecting to the Connecting to the DatabaseDatabase

String path = "jdbc:oracle:thin:";String host = "sol4";String port = "1521";String db = "stud";String login = “sqlUser";String password = “passwd”;String url = path + login + "/" + password +

"@" + host +":" + port + ":" + db;

Class.forName("oracle.jdbc.driver.OracleDriver");Connection con = DriverManager.getConnection(url);

StatementsStatements

1. Statement createStatement()

– returns a new Statement object

2. PreparedStatement

prepareStatement(String)

– returns a new PreparedStatement object

• Both are used to send SQL commands to the DB

• Both are created via the connection object

“prepared” because it already includes the query string

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Statement query methodsStatement query methods

1. executeQuery(String query): for queries

that return a single ResultSet object

(typically select)

2. executeUpdate(String query): for

INSERT, UPDATE, DELETE, and SQL DDL

directives

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CompilationCompilation

• When executing an SQL statement via

JDBC, it is not checked for errors until it

is run (Not checked during compilation)

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executeQueryexecuteQuery

String queryStr = "SELECT * FROM Sailors " +"WHERE Name = 'joe smith'";

Statement stmt = con.createStatement();ResultSet rs = stmt.executeQuery(queryStr);

• The executeQuery method returns a ResultSet object representing the query result.

No semi-colon(;)

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executeUpdateexecuteUpdate

String deleteStr = “DELETE FROM Sailors " +"WHERE sid = 15";

Statement stmt = con.createStatement();int delnum = stmt.executeUpdate(deleteStr);

•executeUpdate returns the number of rows modified

No semi-colon(;)

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PreparedStatement PreparedStatement motivationmotivation

• Suppose we would like to run the query

SELECT * FROM Emp

where name=‘moshe’;

• But we would like to run this for all employees (separately), not only ‘moshe’…

• Could we create a variable instead of ‘moshe’ which would get a different name every time??..

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Prepared StatementsPrepared Statements

• Prepared Statements are used for queries that

are executed many times with possibly

different contents.

• A PreparedStatement object includes the query

and is prepared for execution (precompiled).

• Question marks can be inserted as variables.

-setString(i, value)

-setInt(i, value)The i-th question

mark is set to the

given value.

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PreparedStatement.executeQueryPreparedStatement.executeQuery() ()

String queryStr = "SELECT * FROM Sailors " +"WHERE Name = ? and Rating < ?”;

PreparedStatement pstmt = con.prepareStatement(queryStr);

pstmt.setString(1, “Joe”);pstmt.setInt(2, 8);

ResultSet rs = pstmt.executeQuery();

1st question

mark

Value to insert

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PreparedStatement.executeUpdatePreparedStatement.executeUpdate()()

String deleteStr = “DELETE FROM Boats " +"WHERE Name = ? and Color = ?”;

PreparedStatement pstmt = con.prepareStatement(deleteStr);

pstmt.setString(1, “Fluffy”);pstmt.setString(2, "red");

int delnum = pstmt.executeUpdate();

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• Will this work?

PreparedStatement pstmt = con.prepareStatement(“select * from ?”);

pstmt.setString(1, "Sailors");

No! We may put ? only instead of values

ResultSet (1)ResultSet (1)• A ResultSet is an object which contains the

result of a query - a “table”.

• At most one ResultSet per Statement can be open at the same time(!!).

• A ResultSet maintains a cursor pointing to its current row of data.

• The 'next' method moves the cursor to the next row

• As of JDBC 2.0, scrollable ResultSets are available, which also include ‘previous’, ’first’, ‘last’, etc..

ResultSet (2)ResultSet (2)

• resultSet methods work on the

current row.

• The cursor is positioned before

the first row upon creation.

ResultSet (3)ResultSet (3)

Statement stmt=

con.createStatement();

ResultSet rs =

stmt.executeQuery (

"SELECT * FROM Table1");

while (rs.next()) {

//something…

}

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ResultSet methodsResultSet methods

• Getting the value in some column (for the current row):– getString(int columnNum);

– getString(String columnName);

– getInt(int columnNum);

– getInt(String columnName);

– Etc…

• To check if NULL was returned, you have to use wasNull() on the ResultSet after getting the value.

String s = rs.getString(“column1");

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Example revisitedExample revisited

Statement stmt = con.createStatement();

ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM Table1");

// retrieve and print the values for the current row

while (rs.next()) {

int i = rs.getInt("a");

String s = rs.getString("b");

float f = rs.getFloat("c");

System.out.println("ROW = " + i + " " + s + " " + f);

}

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ResultSetMetaData ResultSetMetaData

ResultSetMetaData rsmd = rs.getMetaData();int numcols = rsmd.getColumnCount();

for (int i = 1 ; i <= numcols; i++) {if (i > 1) System.out.print(",");System.out.print(rsmd.getColumnLabel(i));

}

An object created by the ResultSet which holds information about its columns

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Mapping SQL and Java Mapping SQL and Java Types Types

• SQL and Java data types are not identical

• There are significant variations between the SQL types supported by different database products

• JDBC defines a set of generic SQL type identifiers in the class java.sql.Types

• The driver is responsible for mapping between the DB SQL types and JDBC SQL types

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Some JDBC SQL Types Some JDBC SQL Types

• CHAR

• DATE

• BOOLEAN

• INTEGER

• TINYINT

• VARCHAR

• FLOAT

• DOUBLE

• BIGINT

• BINARY

• CLOB

• BLOB

• ARRAY

• OTHER

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JDBC to Java Type Mapping JDBC to Java Type Mapping

• Each getXXX() and setXXX() method is allowed to get/set columns of certain JDBC types

• For example getDate() is allowed to access columns of types CHAR, VARCHAR, LONGVARCHAR, DATE, and TIMESTAMP

• getDate() is the recommended method to access DATE columns

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Printing Query Output: Printing Query Output: Result Set (2) Result Set (2)

while (rs.next()) {for (int i = 1 ; i <= numcols; i++) {

if (i > 1) System.out.print(",");System.out.print(rs.getString(i));

}System.out.println("");

}

• getString() is allowed to access all simple JDBC types

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Cleaning Up After YourselfCleaning Up After Yourself

• Remember to close the Connections, Statements, PreparedStatements and ResultSets

con.close();stmt.close();pstmt.close();rs.close();

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Dealing With ExceptionsDealing With Exceptions

catch (SQLException e) {//human readable message about the

exceptionSystem.out.println(e.getMessage());

//String describing the reason of the exception

System.out.println(e.getSQLState());//driver-dependent code for the exceptionSystem.out.println(e.getErrorCode());

}

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Transactions in JDBCTransactions in JDBC

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TransactionsTransactions

• Transaction = 2 or more statements which

must all succeed (or all fail) together

• If one fails, the system must reverse all

previous actions

• Aim: don’t leave DB in inconsistent state

halfway through a transaction

• COMMIT = complete transaction

• ROLLBACK = abort

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ExampleExample

• Suppose we want to transfer money from

bank account 13 to account 72:

PreparedStatement pstmt = con.prepareStatement(“UPDATE BankAccount

SET amount = amount + ?

WHERE accountId = ?”);pstmt.setInt(1,-100); pstmt.setInt(2, 13);pstmt.executeUpdate();pstmt.setInt(1, 100); pstmt.setInt(2, 72);pstmt.executeUpdate();

What happens if this update fails?

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Transaction ManagementTransaction Management

• The connection has a state called

AutoCommit mode

• if AutoCommit is true, then every statement

is automatically committed

• if AutoCommit is false, then every statement

is added to an ongoing transaction

• Default: true

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AutoCommitAutoCommit

• If you set AutoCommit to false, you must

explicitly commit or rollback the transaction

using Connection.commit() and

Connection.rollback()

con.setAutoCommit(boolean val)

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ExampleExamplecon.setAutoCommit(false);try { PreparedStatement pstmt = con.prepareStatement(“update BankAccount

set amount = amount + ? where accountId = ?”);

pstmt.setInt(1,-100); pstmt.setInt(2, 13); pstmt.executeUpdate(); pstmt.setInt(1, 100); pstmt.setInt(2, 72); pstmt.executeUpdate(); con.commit();}catch (SQLException e) { con.rollback();}