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Triggers
1
PL/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 week
DECLARE
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.
• 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)
IS
BEGIN
select logon_num
into num
from mylog
where who = person;
END;
/
4
Triggers- 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.
• 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 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.
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)
Create Before-Trigger:
– For every string inserted into sailorName,
turn it into upper case before insertion
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Trigger syntax
CREATE [or REPLACE] TRIGGER trig_name
{BEFORE | AFTER | INSTEAD OF}
Trigger timing
{DELETE | INSERT | UPDATE}
[of column1, column2,…]
Triggering event
[or {DELETE | INSERT | UPDATE]
[of columnA, columnB,…]
on table_name
[FOR EACH ROW]
[WHEN (condition)]
PL/SQL block
Further restricts
when trigger is
fired
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Backing Up Data
create table sailors(
sid
number,
sname VARCHAR2(30),
rating number check(rating <= 10),
age number
);
create table sailors_backup(
who
varchar2(30),
when_chngd date,
sid
number,
old_rating number,
new_rating number
);
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sailors(sid,sname,rating,age);
sailors_backup(who,when_chngd,sid,
old_rating,new_rating);
CREATE or REPLACE TRIGGER backup_trig
AFTER UPDATE of Rating on Sailors
FOR EACH ROW WHEN (old.rating < new.rating)
BEGIN
INSERT INTO sailors_backup
VALUES (USER, SYSDATE, :old.sid,
:old.rating, :new.rating);
END;
/
• 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..)
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Ensuring Upper Case
CREATE or REPLACE TRIGGER sname_trig
BEFORE INSERT or UPDATE of sname
on Sailors
FOR EACH ROW
BEGIN
:new.sname := UPPER(:new.sname);
END;
/
• Q: Why BEFORE Trigger?
• A: You cannot update inserted values after insertion
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Instead Of Trigger
create view sailors_reserves as
select sailors.sname, reserves.bid
from sailors, reserves
where sailors.sid = reserves.sid;
CREATE or REPLACE TRIGGER view_trig
INSTEAD OF INSERT
on sailors_reserves
FOR EACH ROW
BEGIN
INSERT INTO sailors values(:new.sname);
INSERT INTO reserves values(:new.bid);
END;
/
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Statement Trigger
CREATE or REPLACE TRIGGER
no_work_on_shabbat_trig
BEFORE INSERT or DELETE or UPDATE
on reserves
DECLARE
shabbat_exception EXCEPTION;
BEGIN
if (TO_CHAR(sysdate,'DY')='SAT') then
raise shabbat_exception;
end if;
END;
/
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Another example
create 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 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 Triggers
• Can also define triggers for
– logging in and off
– create/drop table events
– system errors
– etc.
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JDBCTM
"Java Database Connectivity"
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Useful JDBC Links
• Getting Started Guide:
http://java.sun.com/j2se/1.5.0/docs/guide/jdbc/g
etstart/GettingStartedTOC.fm.html
• java.sql Package API:
http://java.sun.com/j2se/1.5.0/docs/api/java/sql/
package-summary.html
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Introduction 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 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 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 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)
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JDBC Architecture (1)
Driver
Manager
Application
Driver
DBMS
• DriverManager is provided by Java
Software as part of the Java 2
Platform.
• Drivers are provided by DBMS vendors.
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
Connecting
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 Database
String
String
String
String
String
String
String
path = "jdbc:oracle:thin:";
host = "sol4";
port = "1521";
db = "stud";
login = “sqlUser";
password = “passwd”;
url = path + login + "/" + password +
"@" + host +":" + port + ":" + db;
Class.forName("oracle.jdbc.driver.OracleDriver");
Connection con = DriverManager.getConnection(url);
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Statements
1. Statement createStatement()
– returns a new Statement object
“prepared” because
it already includes
the query string
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
Statement 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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Compilation
• 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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executeQuery
String queryStr =
"SELECT * FROM Sailors " +
"WHERE Name = 'joe smith'";
No semicolon(;)
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery(queryStr);
• The executeQuery method returns a ResultSet
object representing the query result.
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executeUpdate
String deleteStr =
“DELETE FROM Sailors " +
"WHERE sid = 15";
No semicolon(;)
Statement stmt = con.createStatement();
int delnum = stmt.executeUpdate(deleteStr);
• executeUpdate returns the number of rows
modified
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PreparedStatement motivation
• 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 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)
The i-th question
-setInt(i, value)
mark is set to the
given value.
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PreparedStatement.executeQuery()
String queryStr =
"SELECT * FROM Sailors " +
"WHERE Name = ? and Rating < ?”;
PreparedStatement pstmt =
con.prepareStatement(queryStr);
Value to
insert
pstmt.setString(1, “Joe”);
pstmt.setInt(2, 8);
1st question
mark
ResultSet rs = pstmt.executeQuery();
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PreparedStatement.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
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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 methods work on the
current row.
• The cursor is positioned before
the first row upon creation.
ResultSet (3)
Statement stmt=
con.createStatement();
ResultSet rs =
stmt.executeQuery (
"SELECT * FROM Table1");
while (rs.next()) {
//something…
}
ResultSet methods
• Getting the value in some column (for the
String s =
current row):
– getString(int columnNum);
rs.getString(“column1");
– 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.
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Example 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
An object created by the ResultSet which
holds information about its columns
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));
}
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Mapping SQL and Java 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
• CHAR
• DOUBLE
• DATE
• BIGINT
• BOOLEAN
• BINARY
• INTEGER
• CLOB
• TINYINT
• BLOB
• VARCHAR
• ARRAY
• FLOAT
• OTHER
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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:
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 Yourself
• Remember to close the Connections,
Statements, PreparedStatements and
ResultSets
con.close();
stmt.close();
pstmt.close();
rs.close();
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Dealing With Exceptions
catch (SQLException e) {
//human readable message about the exception
System.out.println(e.getMessage());
//String describing the reason of the exception
System.out.println(e.getSQLState());
//driver-dependent code for the exception
System.out.println(e.getErrorCode());
}
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Transactions in JDBC
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Transactions
• 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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Example
• 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();
What happens if this
pstmt.setInt(1, 100);
update fails?
pstmt.setInt(2, 72);
pstmt.executeUpdate();
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Transaction 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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AutoCommit
con.setAutoCommit(boolean val)
• If you set AutoCommit to false, you must
explicitly commit or rollback the transaction
using Connection.commit() and
Connection.rollback()
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Example
con.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();
}
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