1. Lecture 4 Introduction to PL/SQL Procedures & Cursors

2. Overview Overview of PL/SQL Development of a coded block Interacting with an Oracle Database Controlling PL/SQL process flow Cursor handling

3. Embedding SQL SQL is not functionally complete Lacks the full facilities of a programming language variables, flow of control etc. All DBMSs top up functionality by embedding SQL in a procedural language But details vary from one DBMS to another. However, procedures and functions can be ported between systems.

4. PL/SQL - Introduction An Oracle-specific procedural extension to SQL, allowing for modularity, variable declaration, loops and logical constructs. Allows for advanced error handling. Communicates natively with other Oracle database objects. Managed centrally within the Oracle database.

5. Why use PL/SQL? Manage business rules – through middle layer application logic. Generate code for triggers. Generate code for the user interface. Enable database-centric client/server applications.

6. Centralised vs. Decentralised Common copy of executed code – one copy to maintain Multiple copies of executable code on the decentralised system – multiple copies to maintain leading to increase difficulty in maintaining the system Begin : End; Server Begin : End; Begin : End; Begin : End; Server

7. Advantages of using PL/SQL to access Oracle PL/SQL is managed centrally within the database. Code is managed by the DBA, and execution privileges are managed in the same way as with other objects. PL/SQL objects are first-class Oracle DB objects. Easy to read: with modularity features and error handling.

8. Centralised Control Enables the DBA to: specify rules in one place (as a procedure, function, trigger or package in PL/SQL); force user access through the predefined PL/SQL, so users cannot write their own procedural code and use this instead e.g. define security privileges giving users access to table(s) only through a particular procedure.

9. Using PL/SQL as a Programming Language Permits all “flow of control” operations of standard programming languages, e.g. Jumps GOTO Conditions IF-THEN-END IF; IF-THEN-ELSE-END IF Loops LOOP-EXIT; WHEN-END LOOP; FOR-END LOOP; WHILE-END LOOP Allows extraction of data into variables and its subsequent manipulation.

10. Modules in PL/SQL There are 4 types of modules in PL/SQL Procedure – a series of statements which may or may not return a value. Function – a series of statements which must return a single value. Trigger – a series of statements which is executed after an event has triggered a condition. Package – a collection of procedures and functions which has 2 parts: a listing and a body.

11. Use of Data Types Number – used to store any number. Char(size) & varchar2(size) e.g. char(10) – used to store alphanumerical text strings; the char data type will pad the value stored to the full length declared. Date – used to store dates and times. Long – used to store large blocks of text up to 2 gigabytes in length (limited operations)

12. Non-DB Data Types DEC, DECIMAL, REAL, INTEGER, INT – these are numerical data types that are a subset of number. Binary_integer – binary format for number type but can not be stored in database unless converted first. Character – same as char. Boolean – true/false value. Table/record – tables can be used to store the equivalent of an array while records store the variables with composite data types.

13. SQL Scripts A set of commands to run in sequence. Stored as a text file (e.g. using Notepad) on disk and not in the data dictionary. It is accessed by its file name using @ or Start. Script called: Create_lecturer_copy.sql Executed by: SQL> @U:\create_lecturer_copy

14. The SQL Procedure A block of SQL statements stored in the Data Dictionary and called by applications. Satisfies frequently-used or critical application logic. When called, all code within the procedure is executed (unlike packages). Action takes place on the server, not the client. Does not (normally) return a value to the calling program. Not available in Oracle 6 or older. Aids security as DBA may grant access to procedures rather than tables, therefore some users cannot access tables except through a procedure.

15. Building a Procedure: Contents 1. CREATE OR REPLACE command; 2. Object to be created; 3. Name of object; 4. Any variables accessed or imported; 5. Local variables declared; 6. Code block enclosed by BEGIN … END;

16. Create or replace procedure inflation_rise (inf_rate in number) Begin update employee set salary = salary + (salary * inf_rate / 100); commit; End; 1. Create or replace command 2. Object to be created 3. Name of object 4. Any variables accessed or imported 5. Declared local variables 6. Code This procedure is called inflation_rise and uses a variable accessed as inf_rate which is a number, this is passed in when the procedure is used. It simply updates the salary by the rate of inflation.

17. Compiling and Executing Procedures Like any program the code needs to be compiled. @inflation_rise compiles the procedure from a file with this name; makes it available to the data base. Execute inflation_rise executes the procedure. Remember to re-compile a procedure after editing. For ease of use, it is best to write procedures in Notepad, then they can be easily edited and you have a back-up copy.

18. Example CREATE OR REPLACE PROCEDURE validate_customer (v_cust VARCHAR) AS v_countNUMBER; BEGIN SELECT COUNT(*) INTO v_count FROM CUSTOMER WHERE CUST_CODE = v_cust; IF v_count > 0 THEN DBMS_OUTPUT.PUT_LINE(‘customer valid’); ELSE DBMS_OUTPUT.PUT_LINE(‘customer not recognised’); END IF; END; SQL Code Block Variable passed into procedure Local variable used by procedure

19. Cursors in SQL Enables users to loop round a selection of data. Stores data select from a query in a temp area for use when opened. Use complex actions which would not be feasible in standard SQL selection queries

20. Declaring Cursors Declared as a variable in the same way as standard variables. Identified as cursor type. SQL included, e.g. Cursor cur_emp is Select emp_id, surname ‘name’, grade, salary From employee Where regrade is true;

21. Cursors A cursor is a temporary store of data. The data is populated when the cursor is opened. Once opened, the data must be moved from the temporary area to a local variable to be used by the program. These variables must be populated in the same order that the data is held in the cursor. The data set is looped round till an exit clause is reached.

22. Current row Cursor Active set 7369SMITHCLERK 7566JONESMANAGER 7788SCOTTANALYST 7876ADAMSCLERK 7902FORDANALYST Cursor Functions

23. Controlling the Cursor Create a named SQL area Create a named SQL area DECLAREDECLARE Identify the active set Identify the active setOPENOPEN Load the current row into variables Load the current row into variablesFETCHFETCH Test for existing rows Test for existing rows EMPTY? Return to FETCH if rows found Return to FETCH if rows found No Release the active set Release the active set CLOSECLOSE Yes

24. Controlling the Cursor… Open the cursor. Cursor Pointer Fetch a row from the cursor. Cursor Pointer Continue until empty. Cursor Pointer Close the cursor. Cursor

25. Cursor Attributes AttributeType Description %ISOPENBoolean Evaluates to TRUE if the cursor is open %NOTFOUNDBoolean Evaluates to TRUE if the most recent fetch does not return a row %FOUNDBoolean Evaluates to TRUE if the most recent fetch returns a row; logical complement of %NOTFOUND %ROWCOUNTNumberEvaluates to the total number of rows returned so far To obtain status information about a cursor.

26. Create or replace procedure proc_test as v_empidnumber; Cursor cur_sample is Select empid from employee where grade > 4; Begin open cur_sample; loop fetch cur_sample into v_empid; exit when cur_sample%notfound; update employee set salary = salary + 500 where empid = v_empid; end loop; End; Open cursor for use. Loops round each value returned by the cursor Place the value from the cursor into the variable v_empid Stop when no more records are found 25463 12245 55983 12524 98543 Data returned by cursor Declare Cursor

27. Notepad file called: Create_procedures.sql 1) Open SQL*Plus and logon 2) At the prompt enter: @create_procedures You will get a prompt which should say ‘procedure created’ otherwise use SHOW ERRORS to view errors in the code. 3) To run the procedure enter: Execute proc_test 4) If you check your data you should now find that the procedure has run successfully

28. Use of conditions IF statements can be used If Then ….. End if; E.g. Remember to end the IF statement Use of indented code will make it easier to debug!... IF v_ename = 'MILLER' THEN v_job := 'SALESMAN'; v_deptno := 35; v_new_comm := sal * 0.20; END IF;... IF v_ename = 'MILLER' THEN v_job := 'SALESMAN'; v_deptno := 35; v_new_comm := sal * 0.20; END IF;...

29. The %ISOPEN Attribute Can fetch rows only when the cursor is open. Use the %ISOPEN cursor attribute before performing a fetch to test whether the cursor is open. Example IF NOT cur_sample%ISOPEN THEN OPEN cur_sample; END IF; LOOP FETCH cur_sample... IF NOT cur_sample%ISOPEN THEN OPEN cur_sample; END IF; LOOP FETCH cur_sample...

30. DECLARE CURSOR emp_cursor IS SELECTempno, ename FROMemp; emp_recordemp_cursor%ROWTYPE; BEGIN OPEN emp_cursor; LOOP FETCH emp_cursor INTO emp_record;... DECLARE CURSOR emp_cursor IS SELECTempno, ename FROMemp; emp_recordemp_cursor%ROWTYPE; BEGIN OPEN emp_cursor; LOOP FETCH emp_cursor INTO emp_record;... Cursors and Records Process the rows of the active set conveniently by fetching values into a PL/SQL RECORD. Example

31. Cursor FOR Loops FOR record_name IN cursor_name LOOP statement1; statement2;... END LOOP; FOR record_name IN cursor_name LOOP statement1; statement2;... END LOOP; The cursor FOR loop is a shortcut to process cursors. Syntax Implicitly opens, fetches, and closes cursor. The record is implicitly declared.

32. Cursor FOR Loops: An Example Retrieve employees one by one until no more are left: Retrieve employees one by one until no more are left: DECLARE CURSOR emp_cursor IS SELECT ename, deptno FROM emp; BEGIN FOR emp_record IN emp_cursor LOOP -- implicit open and implicit fetch occur IF emp_record.deptno = 30 THEN... END LOOP; -- implicit close occurs END; DECLARE CURSOR emp_cursor IS SELECT ename, deptno FROM emp; BEGIN FOR emp_record IN emp_cursor LOOP -- implicit open and implicit fetch occur IF emp_record.deptno = 30 THEN... END LOOP; -- implicit close occurs END;