Why PLSQL Database vendors usually provide a procedural

Why PL/SQL? • Database vendors usually provide a procedural language as an extension to SQL. – PL/SQL (ORACLE) – Microsoft SQL Server, Sybase provide their languages – similar to PL/SQL • Such a language is needed in order to code various business rules, such as: – “When you sell a widget, total the monthly sale figures, and decrease the widget inventory. ” – “Only a manager can discount blue widgets by 10%. ” • Such rules are impossible to be enforced by just using SQL. • Before PL/SQL, the only was to bundle SQL statements in complex procedural programs running outside the DB server, in order to enforce business rules.

“I’d rather use a ‘real’ language like C!” • While using a “host language” like C can work, more or less, it has some limitations: • The C’s variables store data in different binary format from that of the database. • Hence, each time a value is retrieved from the database into a C variable a conversion is being done, which when multiplied by thousands or millions of occurrences, it can turn to be a big impact. • Also the range of the data types is different. • E. g. where would you store a database INT, which can be 38 digits long? If we are not careful and assign it to a C int or even a long, the value can be truncated. • There are missing datatypes in C, which are present in databases e. g. DATE. • In such cases we use conversions to strings, which are expensive. Even more expensive is then the string manipulation… • A C program running outside the database, which can be invoked only by humans, comes up lacking when we need to protect and secure our data. • Surely, the DBA has some control over who can change some data. So Herman in accounting may receive UPDATE privileges on a receivables table. We might try to control what operations he can perform by programming some business rules in the C program that he uses. Well he is supposed to use it anyway! If he happens to have, say, Microsoft Excel, and he happens to also have connector software to let it talk to Oracle, boom! Herman can bypass all our carefully programmed security checks!

PL/SQL advantages • PL/SQL offers the following advantages: • The PL/SQL variables store data in the same binary format with that of the database. • Hence there is no need for data conversions. • PL/SQL provides the same data types as the database. • There aren’t the C problems related to different datatype ranges or missing datatypes. • A PL/SQL program can run without the human invocation (e. g. triggers). • Such programs can be used to encode business rules protecting and securing our data, but also for encoding other business rules elegantly and in a centralized way.

The first PL/SQL program • You can execute this from the SQLPLUS command prompt: BEGIN DBMS_OUTPUT. PUT_LINE('Hello World!'); END; / • This is called an anonymous block – that is a block without a name. • A block is surrounded by BEGIN, END keywords. • The built-in procedure PUT_LINE, part of the DBMS_OUTPUT package, is being called. This procedure takes a string as input and displays that string on screen. • The / indicates that we are finished. • However, in order to really see the “Hello world” message on the screen, you should set the SQLPLUS environment variable: • SQL> SET SERVEROUTPUT ON

Anonymous blocks • There are several forms an anonymous block can have. Previously we saw one: BEGIN statements END; • Interestingly, a block is just a long PL/SQL statement; notice that the block itself ends with a semicolon. Hence blocks can be nested as we wish. • The second major form of an anonymous block includes a declaration section: DECLARE declarations BEGIN statements END; DECLARE msg VARCHAR(30); --a variable BEGIN msg : = ‘Hello world’; DBMS_OUTPUT. PUT_LINE(msg); END; • The general form of an anonymous block is: DECLARE declarations BEGIN statements [EXCEPTION exception handlers] END; • Exception handlers are statements that will run when certain error conditions occur. • An exception section contains the logic that describes how your block will respond to errors that occur inside the execution section.

• Variables Every variable before being used should be declared in DECLARE section. • PL/SQL data types are a superset of the Oracle data types. I. e. what most of the cases we use is: • VARCHAR 2(n) • INT • NUMBER[(n, m)] • DATE • There also, other types that are only in PL/SQL: • PLS_INTEGER (smaller integer) • BOOLEAN (Can be TRUE, FALSE, NULL) • The syntax for declaring variables: variable_name DATATYPE [CONSTANT] [: = | DEFAULT initial_value] • If the keyword CONSTANT is present the initial value of the variable cannot be changed. • : = or DEFAULT are synonyms in this context for assigning a initial value to the variable. • The initial value should be a literal adhering to the rules for the datatype. • E. g. name VARCHAR 2 : = 'Oracle'; cur_date DATE : = SYSDATE;

Operators • : = Assignment • + Addition • - Subtraction • / Division • * Multiplication • ** Power • AND, OR, NOT Logical operators • = Equality • !=, <>, ~=, ^= Inequality (four variants) • <, > Less than, Greater than • <=, >= • IN • BETWEEN • IS NULL, IS NOT NULL • LIKE (as in SQL for strings) • || Examples: square : = x**2; membership in a set Range test Concatenation of strings square_root : = x**0. 5; order_overdue BOOLEAN : = ship_date > ADD_MONTHS(order_date, 3) OR priority_level(company_id) = 'High'; full_name = 'Chris' || 'Smith'; IF number_of_pages IS NULL THEN DBMS_OUTPUT. PUT_LINE('Unknown'); END;

Conditional logic –IF statement IF condition 1 THEN statements [ELSIF condition 2 THEN] statements [ELSE last_statements] END IF; Examples: IF hourly_wage < 10 THEN hourly_wage : = hourly_wage * 1. 5; ELSE hourly_wage : = hourly_wage * 1. 1; END IF; IF salary BETWEEN 10000 AND 40000 THEN bonus : = 1500; ELSIF salary > 40000 AND salary <= 100000 THEN bonus : = 1000; ELSE bonus : = 0; END IF; Comments: • If none of the conditions are true and there is an ELSE clause, then the last_statements are executed. • The end of the IF statement is “END IF; ” with a space in between. • The “otherwise if” is ELSIF not ELSEIF • You can put parenthesis around boolean expression after the IF and ELSIF but you don’t have to. • You don’t need to put {, } or BEGIN, END to surround several statements between IF and ELSIF/ELSE, or between ELSIF/ELSE and END IF;

Conditional logic –Simple CASE statement CASE selector WHEN expression 1 THEN statements [WHEN expression 2 THEN statements] [ELSE statements] END CASE; • selector can be an expression of any datatype, and it provides the value we are comparing. • expressionn is the expression to test for equality with the selector. • If no WHEN matches the selector value, then the ELSE clause is executed. • If there is no ELSE clause PL/SLQ will implicitly supply: ELSE RAISE CASE_NOT_FOUND; which will terminate the program with an error (if we don’t catch this exception). E. g. DECLARE grade VARCHAR 2(1) : = 'B'; BEGIN CASE grade WHEN 'A' THEN dbms_output. put_line('Excellent'); WHEN 'B' THEN dbms_output. put_line('Very Good'); WHEN 'C' THEN dbms_output. put_line('Good'); WHEN 'D' THEN dbms_output. put_line('Fair'); WHEN 'F' THEN dbms_output. put_line('Poor'); ELSE dbms_output. put_line('No such grade'); END CASE; END; /

Conditional logic –Searched CASE statement • Consider the following IF statement: IF salary BETWEEN 10000 AND 40000 THEN bonus : = 1500; ELSIF salary > 40000 AND salary <= 100000 THEN bonus : = 1000; ELSE bonus : = 0; END IF; • Can we rewrite this IF statement using the previous simple CASE statement? • The answer is No! We need a more powerful statement than the simple CASE – we need the searched CASE • Again, if there is no ELSE clause PL/SLQ will implicitly supply: ELSE RAISE CASE_NOT_FOUND; which will terminate the program with an error (if we don’t catch exception). DECLARE salary INT : = 50000; bonus INT; BEGIN CASE WHEN salary BETWEEN 10000 AND 40000 THEN bonus : = 1500; WHEN salary > 40000 AND salary <= 100000 THEN bonus : = 1000; ELSE bonus : = 0; END CASE; WHEN search_condition 1 THEN statements 1; [WHEN search_condition 2 THEN statements 2; ]. . . [ELSE statements; ] END CASE; dbms_output. put_line('Bonus is ' || bonus); END; /

When CASE is an expression instead… • A CASE statement can also serve as an expression: DECLARE grade CHAR(1) : = 'B'; appraisal VARCHAR 2(20); BEGIN DBMS_OUTPUT. PUT_LINE(appraisal); appraisal : = CASE grade WHEN 'A' THEN 'Excellent' WHEN 'B' THEN 'Very Good' WHEN 'C' THEN 'Good' WHEN 'D' THEN 'Fair' WHEN 'F' THEN 'Poor' ELSE 'No such grade' END; DBMS_OUTPUT. PUT_LINE(appraisal); END; / Observe how the appraisal variable takes the value. • Another example: DECLARE salary INT : = 50000; bonus INT; BEGIN bonus : = CASE WHEN salary BETWEEN 10000 AND 40000 THEN 1500 WHEN salary > 40000 AND salary <= 100000 THEN 1000 ELSE 0 END; dbms_output. put_line('Bonus is ' || bonus); END; / Observe how bonus takes the value. • Notice the absence of the terminators “; ”. • Notice the automatic conversion of bonus into string when we print on the screen. • There is no exception thrown if no WHEN gets matched, and there is missing ELSE. The value returned is NULL.

Iterative Control: LOOP and EXIT Statements • • LOOP statements let you execute a sequence of statements multiple times. There are three forms of LOOP statements: • LOOP, • WHILE-LOOP, and • FOR-LOOP. • LOOP: The simplest form of LOOP statement is the basic (or infinite) loop, which encloses a sequence of statements between the keywords LOOP and END LOOP, as follows: LOOP statements END LOOP; • You can use an EXIT statement to complete the loop. • There are two forms of EXIT statements: EXIT and EXIT-WHEN. • Another example: • E. g. Suppose we wish to categorize salaries according to their number of digits. DECLARE digits INT : = 1; salary INT : = 3005000; temp INT : = salary; BEGIN LOOP temp : = temp / 10; IF temp = 0 THEN EXIT; END IF; --Or we can do: EXIT WHEN temp = 0 digits : = digits + 1; END LOOP; dbms_output. put_line('Number of digits is ' || digits); END; /

WHILE-LOOP WHILE condition LOOP statements END LOOP; DECLARE digits INT : = 0; salary INT : = 3005000; temp INT : = salary; BEGIN WHILE temp > 0 LOOP digits : = digits + 1; temp : = temp / 10; END LOOP; dbms_output. put_line('Number of digits is ' || digits); END; /

FOR Loops • FOR loops iterate over a specified range of integers. FOR counter IN [REVERSE] low. . high LOOP sequence_of_statements END LOOP; • The range is evaluated when the FOR loop is first entered and is never reevaluated. • By default, iteration proceeds upward from the lower bound to the higher bound. However, if you use the keyword REVERSE, iteration proceeds downward from the higher bound to the lower bound. • Nevertheless, you write the range bounds in ascending (not descending) order. • Inside a FOR loop, the loop counter can be referenced like a constant but cannot be assigned values. • What happens if you had declared the loop control variable before?

Background on the example • Let’s build a system that will assist in the cataloging and searching of library books. • For now, we’d like to address two requirements: • Allow the creation of catalog entries for each newly acquired book. • Provide means of counting how many copies of a particular book the library owns. • A simple E/R schema would be: Books Owns Book copies CREATE TABLE books ( isbn VARCHAR 2(13) PRIMARY KEY, title VARCHAR 2(200), summary VARCHAR 2(2000), author VARCHAR 2(200), date_published DATE, page_count NUMBER ); CREATE TABLE book_copies ( barcode_id VARCHAR 2(100) PRIMARY KEY, isbn VARCHAR 2(13) REFERENCES books(isbn) );

Implementing a stored procedure to add a book CREATE OR REPLACE PROCEDURE add_book ( isbn_in IN VARCHAR 2, barcode_id_in IN VARCHAR 2, title_in IN VARCHAR 2, author_in IN VARCHAR 2, page_count_in IN NUMBER, summary_in IN VARCHAR 2 DEFAULT NULL, date_published_in IN DATE DEFAULT NULL ) AS BEGIN /*Check for reasonable inputs*/ IF isbn_in IS NULL THEN RAISE VALUE_ERROR; END IF; • If you don’t set: • show errors • and you have compilation, PL/SQL will just tell you the fact that there are errors, but won’t show you them. INSERT INTO books (isbn, title, summary, author, date_published, page_count) VALUES (isbn_in, title_in, summary_in, author_in, date_published_in, page_count_in); /*if barcode is supplied, put a record in the book_copies table*/ if NOT(barcode_id_in IS NULL) then insert into book_copies (isbn, barcode_id) values(isbn_in, barcode_id_in); end if; END add_book; / show errors

Using the procedure to add a book BEGIN add_book( '1 -56592 -335 -9', '100000002', 'Oracle PL/SQL Programming', 'Feuerstein, Steven, with Bill Pribyl', 987, 'Reference for PL/SQL developers, ' || 'including examples and best practice recommendations. ', TO_DATE('01 -Sep-1997', 'DD-MON-YYYY') ); END; /

Adding a book… • Parameter names: It’s good if you follow the convention to end the IN parameters with a suffix _in. (similarly _out for the OUT parameters, or _inout for the INOUT parameters). • Such naming is not compulsory but helps in avoiding conflicts with the columns names. E. g. • If we didn’t put the _in suffix, then it is hard to read code like this: INSERT INTO books (isbn, title, summary, author, date_published, page_count) VALUES (isbn, title, summary, author, date_published, page_count); • What are those column names or are they PL/SQL variables? In this particular example it turns out that PL/SQL is able to interpret isbn…page_count of the first line as table columns, while in second as PL/SQL variables. • But, what about this: UPDATE Books SET summary = summary WHERE isbn = isbn; • The above won’t work!

Retrieving a book count with a function (contin. ) CREATE OR REPLACE FUNCTION book_copy_qty (isbn_in VARCHAR 2) return NUMBER AS number_of_copies NUMBER : = 0; CURSOR bc_cur IS SELECT count(*) FROM book_copies WHERE isbn = isbn_in; BEGIN IF isbn_in IS NOT NULL THEN OPEN bc_cur; • bc_cur is cursor; we will talk later about them. For the moment consider it as a vehicle for carrying onetuple-at-a-time from the set result of a query. FETCH bc_cur INTO number_of_copies; CLOSE bc_cur; END IF; RETURN number_of_copies; END; / show errors • We can test this function as: set serveroutput on DECLARE how_many INTEGER; BEGIN dbms_output. put_line( 'Number of copies of 1 -56592 -335 -9: ' || book_copy_qty('1 -56592 -335 -9')); END; /

Dropping, viewing stored programs • The REPLACE keyword that we were adding saved us from dropping a procedure or function each time we do a new version of them. However, suppose that we don’t want anymore a procedure or function at all. Then we can drop them: DROP PROCEDURE add_book; DROP FUNCTION book_copy_qty; • A natural question about a stored procedure or function is: Since it’s stored, can we view it? • The answer is yes: SELECT text FROM User_source WHERE name = 'ADD_BOOK' ORDER BY line; • Observe that the name of the procedure or function is stored in upper case.

Stored procedures • The syntax to create a stored procedure is as follows: CREATE [OR REPLACE] PROCEDURE procedure_name [(parameter 1 MODE DATATYPE [DEFAULT expression], parameter 2 MODE DATATYPE [DEFAULT expression], …)] AS [variable 1 DATATYPE; • MODE can be IN for read-only parameters, OUT for writeonly parameters, or IN OUT for both read and write parameters. • The DATATYPE can be any of the types we already have mentioned but without the dimensions, e. g. variable 2 DATATYPE; …] BEGIN statements [EXCEPTION WHEN exception_name THEN statements] END; / • VARCHAR 2, NUMBER, …, but not • VARCHAR 2(20), NUMBER(10, 3)…

Functions • Before, let’s see the syntax to create a stored function: CREATE [OR REPLACE] FUNCTION function_name [(parameter 1 MODE DATATYPE [DEFAULT expression], parameter 2 MODE DATATYPE [DEFAULT expression], …)] RETURN DATATYPE AS • If you omit it mode it will implicitly be IN. • In the header the RETURN DATATYPE is part of the function declaration and it is required. It tells the compiler what datatype to expect when you invoke the function. statements • RETURN inside the executable section is also required. RETURN expression; • If you miss the RETURN clause in the declaration, the program won’t compile. On the other hand, if you miss the RETURN inside the body of the function, the program will execute but at runtime Oracle will give the error ORA-06503: PL/SQL: Function returned without a value. [variable 1 DATATYPE; variable 2 DATATYPE; …] BEGIN [EXCEPTION WHEN exception_name THEN statements] END; /

Creating some data for the examples DECLARE PROCEDURE insert_book_no_complaints (isbn_in IN VARCHAR 2, title_in IN VARCHAR 2, summary_in IN VARCHAR 2, author_in IN VARCHAR 2, date_published_in IN DATE, page_count_in IN NUMBER) IS BEGIN INSERT INTO books (isbn, title, summary, author, date_published, page_count) VALUES (isbn_in, title_in, summary_in, author_in, date_published_in, page_count_in); EXCEPTION WHEN DUP_VAL_ON_INDEX THEN NULL; END; PROCEDURE insert_copy_no_complaints (barcode_id_in IN VARCHAR 2, isbn_in IN VARCHAR 2) IS BEGIN INSERT INTO book_copies (barcode_id, isbn) VALUES (barcode_id_in, isbn_in); EXCEPTION WHEN DUP_VAL_ON_INDEX THEN NULL; END;

BEGIN insert_book_no_complaints ('0 -596 -00180 -0', 'Learning Oracle PL/SQL', 'Beginner''s guide to Oracle''s PL/SQL Programming Language', 'Bill Pribyl with Steven Feuerstein', '29 -Nov-2001', 401); insert_book_no_complaints ('1 -56592 -335 -9', 'Oracle PL/SQL Programming', 'Reference for PL/SQL developers, including examples and best practice ' || 'recommendations. ', 'Feuestein, Steven, and Bill Pribyl', '01 -SEP-1997', 987); insert_book_no_complaints ('1 -56592 -457 -6', 'Oracle PL/SQL Language Pocket Reference', 'Quick-reference guide for Oracle PL/SQL developers. Includes Oracle 8 i ' || 'coverage. ', 'Feuerstein, Steven, Bill Pribyl, Chip Dawes', '01 -APR-1999', 94);

insert_book_no_complaints ('0 -14071 -483 -9', 'The tragedy of King Richard the Third', 'Modern publication of popular Shakespeare historical play in which a ' || 'treacherous royal attempts to steal the crown but dies horseless ' || 'in battle. ', 'Shakespeare, William', '01 -AUG-2000', 158); insert_book_no_complaints ('0 -14 -071415 -4', 'The Tempest', 'Duke and daughter on enchanted island encounters former enemies in this ' || 'comic tale of mystery, love, magic, and (ultimately) forgiveness. ', 'Shakespeare, William', '01 -JAN-1959', 120); insert_book_no_complaints ('0 -672 -31798 -2', 'Sams Teach Yourself PL/SQL in 21 Days, Second Edition', 'Tutorial for Oracle''s procedural language organized around presenting ' || 'language features in a three-week learning schedule. ', 'Gennick, Jonathan, with Tom Luers', '01 -DEC-1999', 692);

insert_book_no_complaints ('0 -07 -882438 -9', 'Oracle PL/SQL Tips and Techniques', 'Voluminous tome presenting tips, techniques, and reference material on ' || 'Oracle''s PL/SQL. ', 'Trezzo, Joseph C. ', '01 -JUL-1999', 942); insert_book_no_complaints ('0 -13 -794314 -8', 'Building Intelligent Databases with Oracle PL/SQL Triggers and Stored ' || 'Procedures-2 nd ed. ', 'Programmer''s guide to PL/SQL, targeted toward building reusable ' || 'components for large Oracle applications. ', 'Owens, Kevin T. ', '01 -JUN-1999', 544); insert_book_no_complaints ('1 -56592 -674 -9', 'Oracle PL/SQL Developer''s Workbook', 'Beginner, intermediate, and advanced exercises designed to test the ' || 'reader''s knowledge of Oracle''s PL/SQL programming language. ', 'Feuerstein, Steven, with Andrew Odewahn', '01 -May-1999', 588);

insert_copy_no_complaints ('92644384', '0 -596 -00180 -0'); insert_copy_no_complaints ('92644385', '0 -596 -00180 -0'); insert_copy_no_complaints ('92644386', '0 -596 -00180 -0'); insert_copy_no_complaints ('89679588', '1 -56592 -335 -9'); insert_copy_no_complaints ('89679589', '1 -56592 -335 -9'); insert_copy_no_complaints ('29846701', '1 -56592 -457 -6'); insert_copy_no_complaints ('29846702', '1 -56592 -457 -6'); insert_copy_no_complaints ('29846703', '1 -56592 -457 -6'); insert_copy_no_complaints ('12983106', '0 -14 -071415 -4'); END; /

Talking in HTML… • CREATE OR REPLACE PROCEDURE show_time Oracle has created mod_plsql Apache module for transparently, “printing” HTML AS the_title VARCHAR 2(30) : = 'What time is it on the from a PL/SQL program on the browser. server? '; • HTP. PRN(expression); • HTP. PRINT(expression); -- prints also a trailing newline. • Let’s print a test page with the current time. http: //athens. cs. uvic. ca: 8080/my_epg_dad/show_time the_time VARCHAR 2(20) : = TO_CHAR(SYSDATE, 'DD-MON-YYYY HH 24: MI'); html VARCHAR 2(200); BEGIN /* construct the HTML in a variable */ html : = '<HTML> <HEAD> <TITLE>' || the_title || '</TITLE> </HEAD> <BODY> It is now: ' || the_time || '</BODY> </HTML>'; /* output to the browser */ HTP. PRINT(html); END; / SHOW ERRORS