Lecture 4

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

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

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. Script called: Create_lecturer_copy.sql 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. 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 • CREATE OR REPLACE command; • Object to be created; • Name of object; • Any variables accessed or imported; • Local variables declared; • Code block enclosed by BEGIN … END;

16. 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. Create or replace command Object to be created Name of object Any variables accessed or imported Declared local variables Code Create or replace procedure inflation_rise (inf_rate in number) Begin update employee set salary = salary + (salary * inf_rate / 100); commit; End;

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. Local variable used by procedure Variable passed into procedure SQL Code Block Example CREATE OR REPLACE PROCEDURE validate_customer (v_cust VARCHAR) AS v_count NUMBER; 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;

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

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. Active set 7369 SMITH CLERK 7566 JONES MANAGER 7788 SCOTT ANALYST 7876 ADAMS CLERK 7902 FORD ANALYST Current row Cursor Cursor Functions

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

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

25. Attribute Type Description %ISOPEN Boolean Evaluates to TRUE if the cursor is open %NOTFOUND Boolean Evaluates to TRUE if the most recent fetch does not return a row %FOUND Boolean Evaluates to TRUE if the most recent fetch returns a row; logical complement of %NOTFOUND %ROWCOUNT Number Evaluates to the total number of rows returned so far Cursor Attributes To obtain status information about a cursor.

26. 25463 12245 55983 12524 98543 Create or replace procedure proc_test as v_empid number; 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; Datareturned by cursor DeclareCursor 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

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 <condition> 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; . . .

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...

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

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

32. Cursor FOR Loops: An Example • Retrieve employees one by one until no more areleft: 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;