1 Triggers

2 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) /

3 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;

4 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; / 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; /

5 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;

6 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.

7 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.

8 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.

9 Example (pseudocode) Create Before-Trigger: –For every string inserted into sailorName, turn it into upper case before insertion

10 Trigger 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

11 Backing 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 );

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

13 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

14 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; /

15 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; /

16 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; /

17 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

18 Additional Types of Triggers Can also define triggers for –logging in and off –create/drop table events –system errors –etc.

19 JDBC TM "Java Database Connectivity"

20 Getting Started Guide: etstart/GettingStartedTOC.fm.html java.sql Package API: package-summary.html Useful JDBC Links

21 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

22 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.

23 Six Steps Load the driver Establish the Connection Create a Statement object Execute a query Process the result Close the connection

24 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)

JDBC Architecture (1) Application DriverManager is provided by Java Software as part of the Java 2 Platform. Drivers are provided by DBMS vendors. Driver Manager DBMS

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

27Connecting 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.

28 Connecting to the Database 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);

Statements 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

30 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

31 Compilation When executing an SQL statement via JDBC, it is not checked for errors until it is run (Not checked during compilation)

32 executeQuery 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(;)

33 executeUpdate 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(;)

34 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??..

35 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) -setInt(i, value) The i-th question mark is set to the given value.

36 PreparedStatement.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(); 1 st question mark Value to insert

37 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();

38 Will this work? PreparedStatement pstmt = con.prepareStatement(“select * from ?”); pstmt.setString(1, "Sailors"); No! We may put ? only instead of values

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… }

42 ResultSet 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");

43 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); }

44 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

45 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

46 Some JDBC SQL Types CHAR DATE BOOLEAN INTEGER TINYINT VARCHAR FLOAT DOUBLE BIGINT BINARY CLOB BLOB ARRAY OTHER

47 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

48 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

49 Cleaning Up After Yourself Remember to close the Connections, Statements, PreparedStatements and ResultSets con.close(); stmt.close(); pstmt.close(); rs.close();

50 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()); }

51 Transactions in JDBC

52 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

53 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(); pstmt.setInt(1, 100); pstmt.setInt(2, 72); pstmt.executeUpdate(); What happens if this update fails?

54 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

55 AutoCommit If you set AutoCommit to false, you must explicitly commit or rollback the transaction using Connection.commit() and Connection.rollback() con.setAutoCommit(boolean val)

56 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(); }