1 / 52

Lecture 5

Lecture 5. Triggers Embedded SQL. Overview. Limitations of Relational Data Model for performing Information Processing Database Triggers in SQL Using Database Triggers for Information Processing within DBMS Restrictions for Database Triggers Embedded SQL JDBC.

aquarius
Download Presentation

Lecture 5

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 5 Triggers Embedded SQL

  2. Overview • Limitations of Relational Data Model for performing Information Processing • Database Triggers in SQL • Using Database Triggers for Information Processing within DBMS • Restrictions for Database Triggers • Embedded SQL • JDBC

  3. Limitations of Relational Data Model • Data Basesvs. Information Systems • DBMS manages data regardless of its usage. • IS processes information with respect to its usage. • Data Model vs. System Architecture • Data model does not give interpretation in terms of the application domain • e.g. relational model, hierarchical model, set model. • IS architecture is developed so that the data can be interpreted as information about a particular applied domain • e.g. HR information, financial information, sales information.

  4. ECA • Event occurs in the data base • e.g. insertion, deletion, update. • Condition is checked • e.g. Is batch complete? Has student passed? • Actions are executed if condition is satisfied • e.g. send batch to supplier, congratulate student.

  5. Extending Information Processing Capabilities of DBMS using Triggers • Processing of database content, performed by the DBMS engine itself, not by the application client • execution of the trigger (Event) • Initiated by a specified condition, depending on the type of the trigger • firing of the trigger (Condition) • All data actions performed by the trigger execute within the same transaction in which the trigger fires, but in a separate session (Action) • Triggers are checked for different privileges as necessary for the processed data. • Cannot contain transaction control statements (COMMIT, SAVEPOINT, ROLLBACK not allowed).

  6. Database Triggers in SQL • Not specified in SQL-92, but standardised in SQL3 (SQL1999) • Available in most commercial DBMS (Oracle, IBM DB2, MS SQL server) and some public domain DBMS (Postgres) • but not in smaller desktop (Oracle Lite) or public domain DBMS (MySQL). • Some vendor DBMS permit native extensions to SQL for specifying the triggers • e.g. PL/SQL in Oracle, Transact SQL in MS SQL Server. • Some DBMS also allow use of general purpose programming languages instead of SQL • e.g. C/C++ in Poet, JAVA in Oracle, C#/VB in SQL Server. • Some DBMS extend the triggers beyond tables • for example also to views as in Oracle.

  7. Types of SQL Triggers • How many times should the trigger body execute when the triggering event takes place? • Per statement: The trigger executes once for the triggering event. This is the default. • For each row: The trigger executes once FOR EACH ROW affected by the triggering event. • When can the trigger be fired? • Relative to the execution of an SQL DML statement (BEFORE or AFTER or INSTEAD OF it) • Exactly in a situation depending on specific system resources (e.g. signal from the system clock, expiring timer, exhausting memory).

  8. Example 2: Monitoring Row Events SQL> UPDATE emp 2 SET sal = sal * 1.1 3 WHERE deptno = 30; Statement and Row Triggers Example 1: Monitoring Statement Events SQL> INSERT INTO dept (deptno, dname, loc) 2 VALUES (50, 'EDUCATION', 'NEW YORK'); Execute only once even if multiple rows affected Execute for each row of the table affected by the event

  9. BEFORE statement trigger AFTER statement trigger Firing Sequence of DatabaseTriggers on a Single Row DEPT table DEPTNO 10 20 30 40 DNAME ACCOUNTING RESEARCH SALES OPERATIONS LOC NEW YORK DALLAS CHICAGO BOSTON BEFORE row trigger AFTER row trigger

  10. BEFORE statement trigger BEFORE row trigger AFTER row trigger BEFORE row trigger AFTER row trigger BEFORE row trigger AFTER row trigger AFTER statement trigger Firing Sequence of Database Triggers on Multiple Rows EMP table EMPNO 7839 7698 7788 ENAME KING BLAKE SMITH DEPTNO 30 30 30

  11. Syntax for creating triggers in SQL • Trigger name - unique within the database schema. • Timing - depends on the order of controlled events (before or after or instead of). • Triggering event – the event which fires the trigger (E). • Filtering condition - checked when the triggering event occurs (C). • Target - table (or view) against which the trigger is fired; they should both be created within the same schema. • Trigger Parameters - parameters used to denote the record columns; preceded by colon • :OLD ,:NEWfor old and new values respectively. • Trigger action - SQL statements, executed when the trigger fires; surrounded by BEGIN ... END (A).

  12. Syntax for Creating Statement Triggers CREATE [OR REPLACE] TRIGGER trigger_name timing event1 [OR event2 OR event3] ON table_name BEGIN SQL or PL/SQL statements; END; The trigger body consisting of SQL statements will be executed only once according to the prescribed timing, when the event1 (event2, event3) occurs against the monitored table in question table_name.

  13. Example: Registering Operations SQL> CREATE TRIGGER increase_salary_trg 2 BEFORE UPDATE OF sal 3 ON emp 4 BEGIN 5 INSERT INTO sal_hist(increased, changedOn) 6 VALUES ('YES', SYSDATE); 7 END; 8 / Trigger name:increase_salary_trg Timing:BEFORE executing the statement Triggering event: UPDATE of sal column Target: emp table Trigger action: INSERT values INTO sal_hist table

  14. Syntax for Creating Row Triggers CREATE [OR REPLACE] TRIGGER trigger_name timing event1 [OR event2 OR event3] ON table_name [REFERENCING OLD AS old | NEW AS new] FOR EACH ROW [WHEN condition] BEGIN SQL or PL/SQL statements; END The trigger body consisting of SQL statements will be executed once for each row affected by event1 (event2, event3) in the table named table_name, subject to the additional condition.

  15. Example: Calculating Derived Columns SQL>CREATE OR REPLACE TRIGGER derive_commission_trg 2 BEFORE UPDATE OF sal ON emp 3 FOR EACH ROW 4 WHEN (new.job LIKE 'SALES%') 5 BEGIN 6 :new.comm := :old.comm * (:new.sal/:old.sal); 7 END; 8 / Trigger name:derive_commission_trg Timing:BEFORE executing the statement Triggering event: UPDATE of sal column Filtering condition: job LIKE'SALES%' Target: emp table Trigger parameters: old, new Trigger action:calculate the new commission to be updated Note: no (colon : ) before new in WHEN

  16. Trigger Execution order 1. Execute all BEFORE STATEMENT triggers. 2. Disable temporarily all integrity constraints on the table. 3. Loop for each row in the table: 3.1 Execute all BEFORE ROW triggers. 3.2 Execute the SQL statement against the row andperform integrity constraint checking of the data. 3.3 Execute all AFTER ROW triggers. 4. Complete deferred integrity constraint checking against the table. 5. Execute all AFTER STATEMENT triggers.

  17. Controlling Triggers using SQL • Disable or Re-enable a database trigger: • Disable or Re-enable all triggers for a table: • Remove a trigger from the database: ALTER TRIGGER trigger_nameDISABLE | ENABLE ALTER TABLE table_name DISABLE | ENABLE ALL TRIGGERS DROP TRIGGER trigger_name

  18. Using Database Triggers for Information Processing • Auditing Table Operations • Each time a table is accessed, auditing information is recorded against it. • Tracking Record Value Changes • Each time a record value is changed the previous value is recorded. • Protecting Database Referential Integrity: if a foreign key points to changing records • Referential integrity must be maintained. • Maintenance of Semantic Integrity • e.g. when the factory is closed, all employees become unemployed. • Storing Derived Data • e.g. the number of items in the trolley should correspond to the current session selection. • Security Access Control • e.g. checking user privileges when accessing sensitive information.

  19. Auditing Table Operations USER_NAME SCOTT SCOTT JONES TABLE_NAME EMP EMP EMP COLUMN_NAME SAL INS 1 0 UPD 1 1 0 DEL 1 0 … continuation MAX_INS 5 5 MAX_UPD 5 5 0 MAX_DEL 5 0

  20. Example: Counting Statement Execution SQL> CREATE OR REPLACE TRIGGER audit_emp 2 AFTER DELETE ON emp 3 FOR EACH ROW 4 BEGIN 5 UPDATE audit_table SET del = del + 1 6 WHERE user_name = USER 7 AND table_name = 'EMP'; 7 END; 8 / Whenever an employee record is deleted from the database, the counter in an audit table registering the number of deleted rows for the current user in system variable USER is incremented.

  21. Example: Tracing Record Value Changes USER_NAME EGRAVINA NGREENBE ID 7950 7844 OLD_LAST_NAME NULL MAGEE NEW_LAST_NAME HUTTON TURNER TIMESTAMP 12-SEP-04 10-AUG-04 … continuation OLD_TITLE NULL CLERK OLD_SALARY NULL 1100 NEW_SALARY 3500 1100 NEW_TITLE ANALYST SALESMAN

  22. Example: Recording Changes SQL>CREATE OR REPLACE TRIGGER audit_emp_values 2 AFTER DELETE OR UPDATE ON emp 3 FOR EACH ROW 4 BEGIN 5 INSERT INTO audit_emp_values (user_name, 6 timestamp, id, old_last_name, new_last_name, 7 old_title, new_title, old_salary, new_salary) 8 VALUES (USER, SYSDATE, :old.empno, :old.ename, 9 :new.ename, :old.job, :new.job, 10 :old.sal, :new.sal); 11 END; 12 / Whenever some details for an employee are deleted or updated, both the previous and new details are recorded in an audit table to allow tracing the history of changes. An insert operation cannot be recorded with this trigger as :old.empno has no value.

  23. Example: Protecting Referential Integrity SQL>CREATE OR REPLACE TRIGGER cascade_updates 2 AFTER UPDATE OF deptno ON dept 3 FOR EACH ROW 4 BEGIN 5 UPDATE emp 6 SET emp.deptno = :new.deptno 7 WHERE emp.deptno = :old.deptno; 8 END 9 / Whenever the department number changes, all employee records for this department will automatically be changed as well, so that the employees will continue to work for the same department.

  24. Restrictions for Database Triggers • Problem: impossible to determine certain values during execution of a sequence of operations belonging to one and the same transaction. • Mutating tables: contain rows which change their values after certain operation and which are used again before the current transaction commits. • Preventing table mutation: • Should not contain rows which are constrained by rows from other changing tables. • Should not contain rows which are updated and read in one and the same operation. • Should not contain rows which are updated and read via other operations during the same transaction.

  25. Example: Mutating Table SQL> CREATE OR REPLACE TRIGGER emp_count 2 AFTER DELETE ON emp 3 FOR EACH ROW 4 DECLARE 5 num INTEGER; 6 BEGIN 7 SELECT COUNT(*) INTO num FROM emp; 8 DBMS_OUTPUT.PUT_LINE(' There are now ' || num || ' employees.'); 9 END; 10 / SQL> DELETE FROM emp 2 WHERE deptno = 30; ERROR at line 1: ORA-04091: table CGMA2.EMP is mutating, trigger/ function may not see it Under the bar is code entered in SQL*Plus which triggers cascade_updates in this case. Triggers are not executed directly.

  26. Example: Mutating Table (fixed) SQL> CREATE OR REPLACE TRIGGER emp_count 2 AFTER DELETE ON emp 3 FOR EACH ROW 4 DECLARE 5 num INTEGER; 6 BEGIN 7 SELECT COUNT(*) INTO num FROM emp; 8 DBMS_OUTPUT.PUT_LINE(' There are now ' || num || ' employees.'); 9 END; 10 / Now the trigger becomes a statement trigger and the EMP table is no longer a mutating table. SQL> DELETE FROM emp WHERE deptno = 30; There are now 13 employees. … There are now 8 employees. 6 rows deleted.

  27. Rules for Good Practice • Rule 1: Do not change data in the primary key, foreign key, or unique key columns of any table. • Rule 2: Do not update records in the same table you read during the same transaction. • Rule 3: Do not aggregate over the same table you are updating. • Rule 4: Do not read data from a table which is updated during the same transaction. • Rule 5: Do not use Data Control Language statements in triggers.

  28. Additional Literature on Triggers • P. Atzeni, S. Ceri, S.Paraboschi and R. Torlone.Database Systems, Chapter 12 “Active Databases”. McGraw-Hill (1999) • Oracle Database Server Documentation. Oracle9i Database Concepts, Chapter 17 “Triggers”. • Oracle Database Server Documentation. Oracle9i Application Developer's Guide – Fundaments, Chapter 15 “Using Triggers”.

  29. Embedded SQL & JDBC Coupling Modes between Data Base and Programming Languages: • Extending the data base language with programming constructs (e.g. PL/SQL). • Extending programming languages with data base constructs. • Persistent programming languages, data base programming languages. • Embedding a data base programming language into a programming language: “Embedded SQL”. • Data base access from the programming language with specialised constructs.

  30. Embedded SQL • Can embed SQL into various languages such as PASCAL, C, C++ and send command lines to data base • Problems with SQL Embedding • Type systems do not fit • Set-oriented vs. individual (scalar) variables • Practical Solution • Mapping of record/attributes to data types of the host language • Iterative processing of the result set by a cursor • Effects on the Host Language • Structure of the host language remains unchanged • Every SQL statement can be embedded • simply prefixed by EXEC SQL • How to communicate between application program and data base?

  31. Development of Embedded SQL application Must have a connection established in order to run

  32. Establishing a Connection Application with embedded SQL: data base connection must be established explicitly. EXEC SQL CONNECT :username IDENTIFIED BY :passwd; username and passwd are host variables of the types CHARor VARCHAR2 • Strings are not allowed! Not supported by the SQL environment Equivalent: EXEC SQL CONNECT :uid; where uid has a value of the form "name/passwd". NOTE: JAVA has problems connecting if the sid of the database has an underscore in the name, e.g. info_db

  33. Host Variables (calling program) • Needed for communication between the data base and the application program: • Output variables for communication of values from the database to the application program Data Base Application • Input variables for the communication of values from the application program to the database.Application Data Base • Data types of the data base and programming language must be compatible.

  34. Handling multiple row return • Queries to the data base may return multiple rows • these will be lost if not handled correctly. • Cursors • Analogous to PL/SQL • Required for processing any result set that may contain more than one row • Cursor operations • DECLARE <cname> CURSOR FOR <SQLstatement> • OPEN <cname> • FETCH <cname> INTO <varlist> • CLOSE <cname> • Error Situations • Cursor not declared or not opened. • No (further) data has been found (incorrect EXIT clause). • Cursor has been closed, and not reopened.

  35. int main() { EXEC SQL BEGIN DECLARE SECTION; char cityName[25]; /* output host var */ int cityEinw; /* output host var */ char* landID = "D"; /* input host var */ short ind1, ind2; /* indicator var */ char* uid = "/"; EXEC SQL END DECLARE SECTION; EXEC SQL CONNECT :uid; /*connect to DB */ EXEC SQL DECLARE StadtCursor CURSOR FOR SELECT Name, Einwohner FROM Stadt WHERE Code = :landID; EXEC SQL OPEN StadtCursor; /* open cursor */ printf("Stadt Einwohner\n"); while (1) {EXEC SQL FETCH StadtCursor INTO :cityName:ind1 , :cityEinw INDICATOR :ind2; if(ind1 != -1 && ind2 != -1) {printf("%s %d \n", cityName, cityEinw); }}; EXEC SQL CLOSE StadtCursor; } /*close cursor */ Example All SQL commands are preceded by EXEC Don’t panic! You are not expected to understand the code … Illustration only!

  36. PL/SQL - blocking of code • Oracle ProC/C++ precompiler supports PL/SQL blocks: • can be used in place of an SQL statement; • reduce communication overhead between client and server. • Frame for communication:EXEC SQL EXECUTE DECLARE ... BEGIN ... END; END-EXEC; • This reduces the need for EXEC commands at the start of each line.

  37. Transactions • Application programming is regarded as a closed transaction, if it is not divided by COMMIT (or ROLLBACK) commands (it happens in isolation and as one block of code). • In Oracle, after leaving a program, COMMIT is executed automatically. • COMMIT is executed automatically before each DDL statement. • The data base connection is shut down by either • EXEC SQL COMMIT RELEASE;or • EXEC SQL ROLLBACK RELEASE; • Transaction handling is covered later in the module.

  38. Additional …. Savepoints • Transactions can be divided by savepoints. • Syntax: EXEC SQL SAVEPOINT <name>; • ROLLBACK to an earlier savepoint deletes all savepoints in between. Exception Handling Mechanism • When you embed code you must handle errors • otherwise the program could crash or hang up. • The WHENEVER statement is the main one used.

  39. WHENEVER statement • Specifies actions to be executed automatically by the DBMS in case of an error: • EXEC SQL WHENEVER <condition> <action>; • <condition> may be: • SQLWARNING: the most recent statement caused a warning different from “no data found”. This corresponds to sqlcode > 0, but <> 1403. • SQLERROR: the most recent statement caused a serious error. This corresponds to sqlcode < 0. • NOT FOUND: SELECT INTO or FETCH did not return any more answer rows. This corresponds to sqlcode 1403.

  40. WHENEVER statement (cont.) • <action> tells the DBMS what to do should an error be encountered during execution, e.g. • CONTINUE: the program continues with the subsequent statement. • DO <proc_name>: invoke a procedure (error handling);DO BREAK for exiting a loop. • GOTO <label>: jump to the given label. • STOP: rollback and exit from the program.

  41. JAVA and Data Bases JAVA is platform-independent: • If a Java Virtual Machine is available, Java programs can be executed. • APIs (Application Programming Interfaces) are collections of classes and interfaces that provide a certain functionality • avoids recoding of common code. JDBC (Java DataBase Connectivity) is an API for data base access: • Interface for (remote) access to a database from Java programs. • Applications can be programmed independently of the underlying DBMS • increased flexibility. • Translates the ODBC idea to Java – established protocol • Common base is the X/Open SQL CLI (Call Level Interface) Standard • conforming to standards improves reliability.

  42. JDBC Architecture

  43. JDBC API Flexible: • Application can be programmed independently from the underlying DBMS. • De facto portability only in the SQL-2 standard (not stored procedures, object-relational features). “Low-level”: • Statements are submitted as strings. • In contrast to Embedded SQL, program variables are not allowed in SQL commands. Under development: • Embedded SQL for JAVA; • Direct mapping of tables and rows to JAVA classes.

  44. JDBC functionality • Establishing a connection to the data base(DriverManager, Connection). • Submission of SQL statements to the database (statement and subclasses) • Processing of the result set(ResultSet)

  45. JDBC: establishing a connection • Invocation of the DriverManager: Connection <name> = DriverManager.getConnection (<jdbc-url>, <user-id>, <passwd>); • Data base is uniquely identified by the JDBC-URL: jdbc:<subprotocol>:<subname> • <subprotocol> identifies the driver and access mechanism • <subname> identifies the data base

  46. jdbc:oracle:<driver-name>: @<IP-Address DB Server>:<Port>:<SID> e.g. String url = 'jdbc:oracle:thin:@132.230.150.11:1521:o901'; Connection conn = DriverManager.getConnection(url,'jdbc_1','jdbc_1'); Returns an opened connection instance, conn. To close the connection: conn.close();

  47. Submitting SQL statements Statement objects: created by invocation of methods of an existing connection: • Statement: simple SQL statement without parameters. • PreparedStatement: precompiled query, query with parameters. • CallableStatement: invocation of a stored PL/SQL procedure.

  48. JDBC Data Types • JDBC stands in between Java (object types) and SQL (predefined types). • java.sql.types defines generic SQL types which are used by JDBC: • If the data types are not supported by SQL-2 then the program cannot use the returned value.

  49. Handling result sets (multiple row return) Information about columns of the result set: • ResultSetMetaData <name> = <result-set>.getMetaData(); creates a ResultSetMetaData object that contains information about the result set:

  50. Prepared Statements PreparedStatement <name> = <connection>.prepareStatement(<string>); • SQL statement <string> is precompiled. • thus, the statement is contained in the state of the object • more efficient than (simple) Statement if it has to be executed several times. • Depending on <string>, one of the (parameterless!) methods • <prepared-statement>.executeQuery() • <prepared-statement>.executeUpdate() • <prepared-statement>.execute() is applicable.

More Related