Homework Assignment. 1. Stored Procedures (SPs)

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1 Homework Assignment In this homework, you will practice how to create database objects such as stored procedures, stored functions, triggers, and events. 1. Stored Procedures (SPs) Stored procedure is a piece of code (procedure) consisting of declarative and procedural SQL statements stored in the catalog of a database that can be activated by calling it from a program, a trigger, or another stored procedure. The following Figure shows how a stored procedure is processed. The left block represents the program from which the procedure is called, the middle block represents the database server, and the right side represents the database and its catalog. The process begins when the procedure is called from the program (step 1). The database server receives this call and finds the matching procedure in the catalog (step 2). Next, the procedure is executed (step 3). After execution, a code is returned indicating that the procedure was processed correctly (step 4). No communication takes place between the database server and the program during the execution of the procedure. Four types of stored programs Stored procedure: can be called from an application that has access to the database Stored function: can be called from SQL statement. A stored function works much like the MySQL built in functions Trigger: is executed in response to an INSERT, UPDATE, or DELETE statement on a specified table Event: is executed at a scheduled time Stored procedures and stored functions are often referred to as stored routines 1

2 Example: a script that creates and calls a stored procedure DROP PROCEDURE IF EXISTS test; -- Change statement delimiter from semicolon to double front slash CREATE PROCEDURE test() DECLARE sum_balance_due_var DECIMAL(9, 2); SELECT SUM(invoice_total - payment_total - credit_total) INTO sum_balance_due_var FROM invoices WHERE vendor_id = 95; -- for testing, the vendor with an ID of 37 has a balance due IF sum_balance_due_var > 0 THEN SELECT CONCAT('Balance due: $', sum_balance_due_var) AS message; ELSE SELECT 'Balance paid in full' AS message; -- Change statement delimiter from semicolon to double front slash CALL test(); The response from the system SQL statements for controlling the flow of execution MySQL provides statements that can be used within scripts to add functionality similar to that provided by procedural programming languages. he flow of execution based on a condition 2

3 REPEAT...UNTIL...END REPEAT //repeats statements while a condition is true DECLARE CURSOR FOR // defines a result set that can be processed by DECLARE...HANDLER //defines a handler that s executed when a stored program encounters an error A SQL statement used within stored programs SELECT: returns a result set to the calling program. Or retrieves data from the database and stores it so it can be processed by the stored program. Example: a stored procedure that displays a message DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() SELECT 'This is a test.' AS message; CALL test(); How to declare and set variables A variable stores a value that can change as the procedure executes. A variable must have a name that is different from the names of any columns used in any SELECT statement within the stored program. To distinguish a variable from a column, you can add a suffix like _var to the variable name. The syntax for declaring a variable DECLARE variable_name data_type [DEFAULT literal_value]; The syntax for setting a variable to a literal value or an expression SET variable_name = literal_value_or_expression; The syntax for setting a variable to a selected value SELECT column_1[, column_2]... INTO variable_name_1[, variable_name_2]... Example: A stored procedure that uses variables DROP PROCEDURE IF EXISTS test; 3

4 CREATE PROCEDURE test() DECLARE max_invoice_total DECIMAL(9,2); DECLARE min_invoice_total DECIMAL(9,2); DECLARE percent_difference DECIMAL(9,4); DECLARE count_invoice_id INT; DECLARE vendor_id_var INT; SET vendor_id_var = 95; SELECT MAX(invoice_total), MIN(invoice_total), COUNT(invoice_id) INTO max_invoice_total, min_invoice_total, count_invoice_id FROM invoices WHERE vendor_id = vendor_id_var; SET percent_difference = (max_invoice_total - min_invoice_total) / min_invoice_total * 100; SELECT CONCAT('$', max_invoice_total) AS 'Maximum invoice', CONCAT('$', min_invoice_total) AS 'Minimum invoice', CONCAT('%', ROUND(percent_difference, 2)) AS 'Percent difference', count_invoice_id AS 'Number of invoices'; CALL test(); The response from the system How to code the IF statements The syntax of the IF statement IF boolean_expression THEN statement_1; [statement_2;]... [ELSEIF boolean_expression THEN statement_1; [statement_2;]...]... [ELSE statement_1; [statement_2;]...] 4

5 You can nest an IF statement within another IF statement or within other SQL statements such as statements for coding loops. You can also code parentheses around the Boolean expressions in an IF statement. (e.g., IF(first_invoice_due_date < NOW()) THEN ) Example DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() DECLARE first_invoice_due_date DATE; SELECT MIN(invoice_due_date) INTO first_invoice_due_date FROM invoices WHERE invoice_total - payment_total - credit_total > 0; IF first_invoice_due_date < NOW() THEN SELECT 'Outstanding invoices overdue!'; ELSEIF first_invoice_due_date = SYSDATE() THEN SELECT 'Outstanding invoices are due today!'; ELSE SELECT 'No invoices are overdue.'; -- the IF statement rewritten as a Searched CASE statement /* CASE WHEN first_invoice_due_date < NOW() THEN SELECT 'Outstanding invoices overdue!' AS Message; WHEN first_invoice_due_date = NOW() THEN SELECT 'Outstanding invoices are due today!' AS Message; ELSE SELECT 'No invoices are overdue.' AS Message; END CASE; */ CALL test(); The response from the system 5

6 SELECT NOW(),CURDATE(),CURTIME(), SYSDATE(); CASE Statements The syntax of the simple CASE statement CASE expression WHEN expression_value_1 THEN statement_1; [statement_2;]... [WHEN expression_value_2 THEN statement_1; [statement_2;]...]... [ELSE statement_1; [statement_2;]...] END CASE; The syntax of the searched CASE statement CASE WHEN boolean_expression THEN statement_1; [statement_2;]... [WHEN boolean_expression THEN statement_1; [statement_2;]...]... [ELSE statement_1; [statement_2;]...] END CASE; You can use a simple CASE statement or a searched CASE statement to execute one or more statements depending on a value that s returned by an expression. Example: DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() 6

7 DECLARE terms_id_var INT; SELECT terms_id INTO terms_id_var FROM invoices WHERE invoice_id = 4; CASE terms_id_var WHEN 1 THEN SELECT 'Net due 10 days' AS Terms; WHEN 2 THEN SELECT 'Net due 20 days' AS Terms; WHEN 3 THEN SELECT 'Net due 30 days' AS Terms; ELSE SELECT 'Net due more than 30 days' AS Terms; END CASE; -- rewritten as a Searched CASE statement /* CASE WHEN terms_id_var = 1 THEN SELECT 'Net due 10 days' AS Terms; WHEN terms_id_var = 2 THEN SELECT 'Net due 20 days' AS Terms; WHEN terms_id_var = 3 THEN SELECT 'Net due 30 days' AS Terms; ELSE SELECT 'Net due more than 30 days' AS Terms; END CASE; */ CALL test(); The response 7

8 Coding Loops While Loop Syntax WHILE boolean_expression DO statement_1; [statement_2;]... END WHILE; Three types of loops: a WHILE loop, a REPEAT loop, and a simple loop. You can use LEAVE statement to go to the end of a loop. You can use the ITERATE statement to go to the beginning of a loop. See the following example: DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() DECLARE i INT DEFAULT 1; DECLARE s VARCHAR(400) DEFAULT ''; -- WHILE loop /* WHILE i < 4 DO SET s = CONCAT(s, 'i=', i, ' '); SET i = i + 1; END WHILE; */ -- REPEAT loop REPEAT SET s = CONCAT(s, 'i=', i, ' '); SET i = i + 1; UNTIL i = 4 END REPEAT; -- LOOP with LEAVE statement /* testloop : LOOP SET s = CONCAT(s, 'i=', i, ' '); SET i = i + 1; IF i = 4 THEN LEAVE testloop; 8

9 END LOOP testloop; */ SELECT s AS message; CALL test(); Using a CURSOR By default, SQL statements work with an entire result set rather than individual rows. However, sometimes, you may need to work with the data in a result set one row at a time. To do that, CURSOR is used. The syntax for using a cursor Declare a cursor DECLARE cursor_name CURSOR FOR select_statement; Declare an error handler for when no rows are found in the cursor DECLARE CONTINUE HANDLER FOR NOT FOUND handler_statement; Open the cursor OPEN cursor_name; Get column values from the row and store them in a series of variables FETCH cursor_name INTO variable1 [, variable2][, variable3]...; Close the cursor CLOSE cursor_name; Example: DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() 9

10 DECLARE invoice_id_var INT; DECLARE invoice_total_var DECIMAL(9,2); DECLARE row_not_found TINYINT DEFAULT FALSE; DECLARE update_count INT DEFAULT 0; DECLARE invoices_cursor CURSOR FOR SELECT invoice_id, invoice_total FROM invoices WHERE invoice_total - payment_total - credit_total > 0; DECLARE CONTINUE HANDLER FOR NOT FOUND SET row_not_found = TRUE; OPEN invoices_cursor; WHILE row_not_found = FALSE DO FETCH invoices_cursor INTO invoice_id_var, invoice_total_var; IF invoice_total_var > 1000 THEN UPDATE invoices SET credit_total = credit_total + (invoice_total *.1) WHERE invoice_id = invoice_id_var; SET update_count = update_count + 1; END WHILE; CLOSE invoices_cursor; SELECT CONCAT(update_count, ' row(s) updated.'); CALL test(); Declaring a condition handler Commonly used MySQL error codes 10

11 Built-in named conditions NOT FOUND occurs when a program attempts to use a FETCH statement or a SELECT statement to retrieve data and no data is found SQLEXCEPTION Occurs when any error condition other than the NOT FOUND condition occurs SQLWARNING Occurs when any error condition other than the NOT FOUND condition occurs or when any warning messages occur The syntax for declaring a condition handler DECLARE {CONTINUE EXIT} HANDLER FOR { mysql_error_code SQLSTATE sqlstate_code named_condition } handler_actions; How to declare a condition handler for a MySQL error code DECLARE CONTINUE HANDLER FOR 1329 SET row_not_found = TRUE How to declare a condition handler for a SQLSTATE code DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET row_not_found = TRUE How to declare a condition handler for a named condition DECLARE CONTINUE HANDLER FOR NOT FOUND SET row_not_found = TRUE To continue execution when an error occurs, use the CONTINUE keyword. To exit the current block of code when an error occurs, use the EXIT keyword. For a complete list of the MySQL error codes and their corresponding SQLSTATE codes, you can search the MySQL reference manual for Server Error Codes. 11

12 Example: a stored procedure that does not handle errors DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() INSERT INTO general_ledger_accounts VALUES (130, 'Cash'); SELECT '1 row was inserted.'; CALL test(); The response A stored procedure that uses a CONTINUE handler DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() DECLARE duplicate_entry_for_key INT DEFAULT FALSE; DECLARE CONTINUE HANDLER FOR 1062 SET duplicate_entry_for_key = TRUE; INSERT INTO general_ledger_accounts VALUES (130, 'Cash'); IF duplicate_entry_for_key = TRUE THEN SELECT 'Row was not inserted - duplicate key encountered.' AS message; ELSE SELECT '1 row was inserted.' AS message; 12

13 CALL test(); Response from the system Example: a stored procedure that uses an EXIT handler to handle an error DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() DECLARE duplicate_entry_for_key INT DEFAULT FALSE; DECLARE EXIT HANDLER FOR 1062 SET duplicate_entry_for_key = TRUE; INSERT INTO general_ledger_accounts VALUES (130, 'Cash'); SELECT '1 row was inserted.' AS message; END; IF duplicate_entry_for_key = TRUE THEN SELECT 'Row was not inserted - duplicate key encountered.' AS message; CALL test(); The response from the system 13

14 Example: a stored procedure that uses a named condition to handle all errors DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() DECLARE sql_error TINYINT DEFAULT FALSE; DECLARE EXIT HANDLER FOR SQLEXCEPTION SET sql_error = TRUE; INSERT INTO general_ledger_accounts VALUES (130, 'Cash'); SELECT '1 row was inserted.' AS message; END; IF sql_error = TRUE THEN SHOW ERRORS; CALL test(); The response SHOW ERRORS statement returns a result set that contains information about the errors that resulted from the last statement in the current session. Multiple condition handlers can be declared for a single stored program. If you do that, the most specific error handlers are executed first and the least specific error handlers are executed last. The MySQL error codes and the NOT FOUND condition identify specific errors. The SQLSTATE codes identify less specific ANSI-standard errors. And the SQLEXCEPTION and SQLWARNING conditions identify general errors. Example: a stored procedure that uses multiple condition handlers 14

15 DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() DECLARE duplicate_entry_for_key INT DEFAULT FALSE; DECLARE column_cannot_be_null INT DEFAULT FALSE; DECLARE sql_exception INT DEFAULT FALSE; DECLARE EXIT HANDLER FOR 1062 SET duplicate_entry_for_key = TRUE; DECLARE EXIT HANDLER FOR 1048 SET column_cannot_be_null = TRUE; DECLARE EXIT HANDLER FOR SQLEXCEPTION SET sql_exception = TRUE; INSERT INTO general_ledger_accounts VALUES (NULL, 'Test'); SELECT '1 row was inserted.' AS message; END; IF duplicate_entry_for_key = TRUE THEN SELECT 'Row was not inserted - duplicate key encountered.' AS message; ELSEIF column_cannot_be_null = TRUE THEN SELECT 'Row was not inserted - column cannot be null.' AS message; ELSEIF sql_exception = TRUE THEN SHOW ERRORS; CALL test(); Transactions A transaction is a group of SQL statements that are combined in a single logical unit of task. By default, a MySQL session uses autocommit mode, which automatically automatically commits INSERT, UPDATE, and DELETE statements. 15

16 When to use transactions When you code two or more INSERT, UPDATE, or DELETE statements that affect related data. When you move rows from one table to another table by using INSERT and DELETE statements When data integrity is violated by INSERT, UPDATE, or DELETE statement. To start a transaction, use the START TRANSACTION statement. This turns off autocommit mode until the statements in the transaction are committed or rolled back. To commit the changes, code a COMMIT statement. To roll back the change, use a ROLLBACK statement. MySQL automatically commits changes after a DDL statement such as a CREATE TABLE statement. You should not code DDL statement within a transaction. Transactions are often coded within stored procedures. The following example shows a stored procedure that runs three INSERT statements as a transaction. How to create stored procedures that accept parameters Syntax of Stored Procedure (SP) CREATE PROCEDURE procedure_name ( [parameter_name_1 data_type] [, parameter_name_2 data_type]... ) sql_block Example: A SP that updates a table DROP PROCEDURE IF EXISTS update_invoices_credit_total; CREATE PROCEDURE update_invoices_credit_total ( invoice_id_param INT, credit_total_param DECIMAL(9,2) ) DECLARE sql_error INT DEFAULT FALSE; DECLARE CONTINUE HANDLER FOR SQLEXCEPTION SET sql_error = TRUE; START TRANSACTION; UPDATE invoices 16

17 SET credit_total = credit_total_param WHERE invoice_id = invoice_id_param; IF sql_error = FALSE THEN COMMIT; ELSE ROLLBACK; -- Use the CALL statement CALL update_invoices_credit_total(56, 200); SELECT invoice_id, credit_total FROM invoices WHERE invoice_id = 56; The system responses credit_total was updated from 0.00 to Use the CALL statement within a stored procedure DROP PROCEDURE IF EXISTS test; CREATE PROCEDURE test() CALL update_invoices_credit_total(56, 300); CALL test(); SELECT invoice_id, credit_total FROM invoices WHERE invoice_id = 56; -- Reset data to original value CALL update_invoices_credit_total(56, 0); 17

18 SELECT invoice_id, credit_total FROM invoices WHERE invoice_id = 56; The syntax for declaring input and output parameters Input parameters accept values that are passed from the calling program. These values cannot be changed by the body of the stored procedure. By default, parameters are defined as input parameters. As a result, the IN keyword is optional for identifying input parameters. Output parameters store values that are passed back to the calling program. These values must be set by the body of the stored procedure. To identify an output parameter, you must use the OUT keyword. INOUT parameters can store an initial value that is passed from the calling program. The body of the SP can change this parameter. When you work with OUT parameters or INOUT parameters, the calling program typically passes a user variable to the parameter list. [IN OUT INOUT] parameter_name data_type Example: DROP PROCEDURE IF EXISTS update_invoices_credit_total; CREATE PROCEDURE update_invoices_credit_total ( IN invoice_id_param INT, IN credit_total_param DECIMAL(9,2), INOUT update_count INT ) DECLARE sql_error INT DEFAULT FALSE; DECLARE CONTINUE HANDLER FOR SQLEXCEPTION SET sql_error = TRUE; START TRANSACTION; UPDATE invoices SET credit_total = credit_total_param WHERE invoice_id = invoice_id_param; IF sql_error = FALSE THEN SET update_count = 1; COMMIT; ELSE 18

19 SET update_count = 0; ROLLBACK; = 0; CALL update_invoices_credit_total(56, CALL update_invoices_credit_total(56, SELECT CONCAT('row_count: AS update_count; The response You can provide a default value for a parameter so that if the calling program passes a null value for the parameter, the default value is used instead. To set a default value for a parameter, you can use an IF statement to check if the parameter contains a null value. If it does, you can assign a default value to the parameter. It is a good programming practice to write SP with a list of parameters that require values first followed by parameters that allow null values. Example: DROP PROCEDURE IF EXISTS update_invoices_credit_total; CREATE PROCEDURE update_invoices_credit_total ( invoice_id_param INT, credit_total_param DECIMAL(9,2) ) DECLARE sql_error INT DEFAULT FALSE; DECLARE CONTINUE HANDLER FOR SQLEXCEPTION SET sql_error = TRUE; -- Set default values for NULL values 19

20 IF credit_total_param IS NULL THEN SET credit_total_param = 100; START TRANSACTION; UPDATE invoices SET credit_total = credit_total_param WHERE invoice_id = invoice_id_param; IF sql_error = FALSE THEN COMMIT; ELSE ROLLBACK; -- call with param CALL update_invoices_credit_total(56, 200); SELECT invoice_id, credit_total FROM invoices WHERE invoice_id = 56; -- call without param CALL update_invoices_credit_total(56, NULL); SELECT invoice_id, credit_total FROM invoices WHERE invoice_id = 56; -- reset data CALL update_invoices_credit_total(56, 0); SELECT invoice_id, credit_total FROM invoices WHERE invoice_id = 56; Validate Parameters Data validation validates the parameters in a SP before using them. The SIGNAL statement raises an error. When an error occurs, SQLSTATE code must be signaled. You may optionally specify an error message or MySQL error number. When an error is raised, MySQL returns the error to the caller in the same way that it returns errors that are raised by the database engine. The calling program can handle the error. The syntax of the SIGNAL statement SIGNAL SQLSTATE [VALUE] sqlstate_value [SET MESSAGE_TEXT = message [, MYSQL_ERRNO = mysql_error_number]] 20

21 Example: DROP PROCEDURE IF EXISTS update_invoices_credit_total; CREATE PROCEDURE update_invoices_credit_total ( invoice_id_param INT, credit_total_param DECIMAL(9,2) ) -- Validate paramater values IF credit_total_param < 0 THEN UPDATE `The credit_total column must be greater than or equal to 0.` SET x = 'This UPDATE statement raises an error'; ELSEIF credit_total_param >= 1000 THEN SIGNAL SQLSTATE '22003' SET MESSAGE_TEXT = 'The credit_total column must be less than 1000.', MYSQL_ERRNO = 1264; -- Set default values for parameters IF credit_total_param IS NULL THEN SET credit_total_param = 100; UPDATE invoices SET credit_total = credit_total_param WHERE invoice_id = invoice_id_param; CALL update_invoices_credit_total(56, NULL); CALL update_invoices_credit_total(56, -100); CALL update_invoices_credit_total(56, 1000); CALL update_invoices_credit_total(56, 0); SELECT invoice_id, credit_total FROM invoices WHERE invoice_id = 56; The response 21

22 Example: a stored procedure that validates the data in a new invoice If the data for each of the columns of the row is valid, the procedure executes an INSERT statement to insert the row. Otherwise, the procedure or database engine raises an error and exits the procedure. DROP PROCEDURE IF EXISTS insert_invoice; CREATE PROCEDURE insert_invoice ( vendor_id_param INT, invoice_number_param VARCHAR(50), invoice_date_param DATE, invoice_total_param DECIMAL(9,2), terms_id_param INT, invoice_due_date_param DATE ) DECLARE terms_id_var INT; DECLARE invoice_due_date_var DATE; DECLARE terms_due_days_var INT; -- Validate paramater values IF invoice_total_param < 0 THEN SIGNAL SQLSTATE '22003' SET MESSAGE_TEXT = 'The invoice_total column must be a positive number.', MYSQL_ERRNO = 1264; ELSEIF invoice_total_param >= THEN SIGNAL SQLSTATE '22003' SET MESSAGE_TEXT = 'The invoice_total column must be less than 1,000,000.', MYSQL_ERRNO = 1264; -- Set default values for parameters IF terms_id_param IS NULL THEN SELECT default_terms_id INTO terms_id_var FROM vendors WHERE vendor_id = vendor_id_param; ELSE SET terms_id_var = terms_id_param; 22

23 IF invoice_due_date_param IS NULL THEN SELECT terms_due_days INTO terms_due_days_var FROM terms WHERE terms_id = terms_id_var; SELECT DATE_ADD(invoice_date_param, INTERVAL terms_due_days_var DAY) INTO invoice_due_date_var; ELSE SET invoice_due_date_var = invoice_due_date_param; INSERT INTO invoices (vendor_id, invoice_number, invoice_date, invoice_total, terms_id, invoice_due_date) VALUES (vendor_id_param, invoice_number_param, invoice_date_param, invoice_total_param, terms_id_var, invoice_due_date_var); -- test CALL insert_invoice(34, 'ZXA-080', ' ', , 3, ' '); CALL insert_invoice(34, 'ZXA-082', ' ', , NULL, NULL); -- this statement raises an error CALL insert_invoice(34, 'ZXA-083', ' ', , NULL, NULL); -- clean up SELECT * FROM invoices WHERE invoice_id >= 115; DELETE FROM invoices WHERE invoice_id >= 115; The response User variable is a special type of MySQL variable that s globally available to the current user. A user variable is only available to the current user and cannot be seen or accessed by other users. A user variable is available as long as the user remains connected to the server, but it is reset when the user disconnects. A user variable can store various types including string, numeric, and date/time types. However, you do not need to declare a data type for a user variable. A user variable is available from statements coded both inside and outside of stored programs. The syntax for setting a user variable 23

24 = expression Example: Two SPs that work with the same user variable DROP PROCEDURE IF EXISTS set_global_count; DROP PROCEDURE IF EXISTS increment_global_count; CREATE PROCEDURE set_global_count ( count_var INT ) = count_var; CREATE PROCEDURE increment_global_count() + 1; CALL set_global_count(100); CALL increment_global_count(); AS count_var; The response Work with dynamic SQL Dynamic SQL is often used to build complex WHERE clauses that depende on multiple search conditions that may or may not be specified by the user. You can use a SP to build a string variable that contains a SQL statement. Then, you can use the PREPARE, EXECUTE, and DEALLOCATE statements to execute the statement contained in the string. Example: a SP that uses dynamic SQL 24

25 DROP PROCEDURE IF EXISTS select_invoices; CREATE PROCEDURE select_invoices ( min_invoice_date_param DATE, min_invoice_total_param DECIMAL(9,2) ) DECLARE select_clause VARCHAR(200); DECLARE where_clause VARCHAR(200); SET select_clause = "SELECT invoice_id, invoice_number, invoice_date, invoice_total FROM invoices "; SET where_clause = "WHERE "; IF min_invoice_date_param IS NOT NULL THEN SET where_clause = CONCAT(where_clause, " invoice_date > '", min_invoice_date_param, "'"); IF min_invoice_total_param IS NOT NULL THEN IF where_clause!= "WHERE " THEN SET where_clause = CONCAT(where_clause, "AND "); SET where_clause = CONCAT(where_clause, "invoice_total > ", min_invoice_total_param); IF where_clause = "WHERE " THEN = select_clause; ELSE = CONCAT(select_clause, where_clause); PREPARE select_invoices_statement EXECUTE select_invoices_statement; DEALLOCATE PREPARE select_invoices_statement; 25

26 CALL select_invoices(' ', 100); CALL select_invoices(' ', NULL); CALL select_invoices(null, 1000); CALL select_invoices(null, NULL); variable at runtime with parameters inserted Drop a SP SELECT invoice_id, invoice_number, invoice_date, invoice_total FROM invoices WHERE invoice_date > ' ' AND invoice_total > 100 The syntax of the DROP PROCEDURE statement DROP PROCEDURE [IF EXISTS] procedure_name 2. Stored Functions (SFs) A stored function (SF), which is also called a user defined function (UDF) or just a function, is an executable database object that contains a block of procedural SQL code. With MySQL, you can only create scalar functions which return a single value. Other DBMS SF may return a result set. To identify the data type that is returned by a function, you use RETURNS keyword in the declaration for the function. Then, in the body of the SF, RETURN keyword is used to specify the value that is returned. RETURN keyword is not allowed in a SP. A function can accept input parameters that work like the input parameters for a stored procedure. A function cannot make changes to the database such as executing an INSERT, UPDATE, or DELETE statement. To call a stored function, you can use it in any expression. Example: DROP FUNCTION IF EXISTS get_vendor_id; CREATE FUNCTION get_vendor_id ( vendor_name_param VARCHAR(50) ) RETURNS INT DECLARE vendor_id_var INT; SELECT vendor_id INTO vendor_id_var 26

27 FROM vendors WHERE vendor_name = vendor_name_param; RETURN(vendor_id_var); SELECT invoice_number, invoice_total FROM invoices WHERE vendor_id = get_vendor_id('ibm'); The response Example: a function that calculates balance due DROP FUNCTION IF EXISTS get_balance_due; CREATE FUNCTION get_balance_due ( invoice_id_param INT ) RETURNS DECIMAL(9,2) DECLARE balance_due_var DECIMAL(9,2); SELECT invoice_total - payment_total - credit_total INTO balance_due_var FROM invoices WHERE invoice_id = invoice_id_param; RETURN balance_due_var; SELECT vendor_id, invoice_number, get_balance_due(invoice_id) AS balance_due FROM invoices WHERE vendor_id = 37; 27

28 To display SPs and SFs in MySQL Workbench 28

29 SHOW PROCEDURE STATUS; Display information about the SPs. SHOW FUNCTION STATUS; Display information about the SFs. 3. Create Triggers Triggers can be executed before or after an INSERT, UPDATE, or DELETE statement is executed on a table. As a result, they provide a powerful way to enforce data consistency. A trigger is a named block of code that executes or fires (in response to an INSERT, UPDATE, or DELETE statement). A trigger can be fired before or after INSERT, UPDATE, or DELETE statement. You must specify a FOR EACH ROW clause. This creates a row-level trigger that fires once for each row that is modified. You can use the OLD and NEW keywords to get and set the values for the columns that are stored in the old row and the new row. The syntax of the CREATE TRIGGER statement CREATE TRIGGER trigger_name {BEFORE AFTER} {INSERT UPDATE DELETE} ON table_name 29

30 FOR EACH ROW sql_block Example: a trigger that corrects mixed-case state names DROP TRIGGER IF EXISTS vendors_before_update; CREATE TRIGGER vendors_before_update BEFORE UPDATE ON vendors FOR EACH ROW SET NEW.vendor_state = UPPER(NEW.vendor_state); #An update statement that fires the trigger UPDATE vendors SET vendor_state = 'wi' WHERE vendor_id = 1; SELECT vendor_name, vendor_state FROM vendors WHERE vendor_id = 1; Example: a trigger that validates line item amounts DROP TRIGGER IF EXISTS invoices_before_update; CREATE TRIGGER invoices_before_update BEFORE UPDATE ON invoices FOR EACH ROW DECLARE sum_line_item_amount DECIMAL(9,2); 30

31 SELECT SUM(line_item_amount) INTO sum_line_item_amount FROM invoice_line_items WHERE invoice_id = NEW.invoice_id; IF sum_line_item_amount!= NEW.invoice_total THEN SIGNAL SQLSTATE 'HY000' SET MESSAGE_TEXT = 'Line item total must match invoice total.'; #Update statement that fires the trigger UPDATE invoices SET invoice_total = 600 WHERE invoice_id = 100; SELECT invoice_id, invoice_total, credit_total, payment_total FROM invoices WHERE invoice_id = 100; The system response Triggers can be used to enforce rules for data consistency that cannot be enforced by constraints How to create an AFTER trigger Triggers are commonly used to store information about actions that occur in a database so these actions can be reviewed later. In particular, AFTER triggers are used to store information about a statement after it executes. Example: -- A statement that creates an audit table DROP TABLE if exists invoices_audit; CREATE TABLE invoices_audit ( vendor_id INT NOT NULL, invoice_number VARCHAR(50) NOT NULL, 31

32 invoice_total DECIMAL(9,2) NOT NULL, action_type VARCHAR(50) NOT NULL, action_date DATETIME NOT NULL ); DROP TRIGGER IF EXISTS invoices_after_insert; DROP TRIGGER IF EXISTS invoices_after_delete; -- two AFTER triggers that insert rows into the audit table CREATE TRIGGER invoices_after_insert AFTER INSERT ON invoices FOR EACH ROW INSERT INTO invoices_audit VALUES (NEW.vendor_id, NEW.invoice_number, NEW.invoice_total, 'INSERTED', NOW()); CREATE TRIGGER invoices_after_delete AFTER DELETE ON invoices FOR EACH ROW INSERT INTO invoices_audit VALUES (OLD.vendor_id, OLD.invoice_number, OLD.invoice_total, 'DELETED', NOW()); -- an INSERT statement that causes the first trigger to fire INSERT INTO invoices VALUES (115, 34, 'ZXA-080', ' ', , 0, 0, 3, ' ', NULL); -- a DELETE statement that causes the second trigger to fire DELETE FROM invoices WHERE invoice_id = 115; SELECT * FROM invoices_audit; The first CREATE TRIGGER statement creates a trigger that executes after an INSERT statement is executed on the Invoices table. This trigger inserts the new values for the vendor_id, invoice_number, invoice_total, string value INSERTED, current date and time. The second trigger works similarly. The response 32

33 View or drop triggers List all triggers in the current database SHOW TRIGGERS; List all triggers in the specified database SHOW TRIGGERS IN ap; List all triggers that begin with ven% SHOW TRIGGERS IN ap LIKE 'ven%'; A statement that drops a trigger only if it exists DROP TRIGGER IF EXISTS vendors_before_update; 4. Events An event, or scheduled event, is a named block of code that executes, or fires, according to the event scheduler. By default the event scheduler is off. As a result, before you begin working with events, you need to turn on the event scheduler. If you do not need to use events, you should turn the event scheduler off to save system resources. Check if the event scheduler is on SHOW VARIABLES LIKE 'event_scheduler'; 33

34 Events are procedural database objects that MySQL invokes at the scheduled time. An event can be invoked only once. An event can be started periodically (e.g., every Sunday at 4:00 AM). MySQL keeps a schedule that tells when events must be started. Events are used for the following cases: Events can close accounts: at end of each month or each year, for example, many accounting departments need to close their accounts. Events can turn database indicators on or off. For example, consider an airplane company database. When a flight has departed, no more reservations can be made for that flight. The column CLOSED in the FLIGHTS table must be set to YES. You can schedule an event to automatically start 20 minutes after the flight s planned departure time to complete the task. Backup database to files Example: -- a statement to check if the event scheduler is on SHOW VARIABLES LIKE 'event_scheduler'; DROP EVENT IF EXISTS one_time_delete_audit_rows; DROP EVENT IF EXISTS monthly_delete_audit_rows; CREATE EVENT one_time_delete_audit_rows ON SCHEDULE AT NOW() + INTERVAL 1 MONTH DO DELETE FROM invoices_audit WHERE action_date < NOW() - INTERVAL 1 MONTH LIMIT 100; CREATE EVENT monthly_delete_audit_rows ON SCHEDULE EVERY 1 MONTH STARTS ' :00:00' DO DELETE FROM invoices_audit WHERE action_date < NOW() - INTERVAL 1 MONTH LIMIT 100; SHOW EVENTS; SHOW EVENTS IN ap; SHOW EVENTS IN ap LIKE 'mon%'; 34

35 ALTER EVENT monthly_delete_audit_rows DISABLE; ALTER EVENT monthly_delete_audit_rows ENABLE; ALTER EVENT one_time_delete_audit_rows RENAME TO one_time_delete_audits; DROP EVENT monthly_delete_audit_rows; DROP EVENT IF EXISTS monthly_delete_audit_rows; 35