Multicolumn index, stored procedure, stored function, trigger, event, and transaction examples

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1 Multicolumn index, stored procedure, stored function, trigger, event, and transaction examples Multicomumn index example use company; desc employee; select * from employee; explain extended select * from employee; explain extended select * from employee where SSN= ; select count(*) from employee; explain extended select * from employee; alter table employee add index name(lname, fname); show indexes from employee; explain extended select * from employee where fname = 'Joe'; explain extended select * from employee where lname = 'Montana'; Stored Procedures 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. 1

2 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 Example: a script that creates and calls a stored procedure USE company; DROP PROCEDURE IF EXISTS test; -- Change statement delimiter from semicolon to double front slash DELIMITER // CREATE PROCEDURE test() BEGIN DECLARE sumofsalaries DECIMAL(9, 2); SELECT SUM(salary) INTO sumofsalaries FROM employee WHERE dno = 1; -- for testing, the vendor with an ID of 37 has a balance due IF sumofsalaries >= 0 THEN 2

3 SELECT CONCAT('Total salaries paid to the employees who work for department 1: $', sumofsalaries) AS message; ELSE SELECT 'No one works for the department 1' AS message; END IF; END// -- Change statement delimiter from semicolon to double front slash DELIMITER ; select count(*), salary from employee where dno = 1; CALL test(); Example: a script that creates and calls a stored function USE company; DROP FUNCTION IF EXISTS getnumberofemployees; delimiter // create function getnumberofemployees(dnumber int) returns int begin declare result int; select count(*) into result from employee where employee.dno = dnumber; return result; end; // delimiter ; SELECT getnumberofemployees(5); 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. 3

4 Example: a script that creates and calls a trigger USE company; DROP TRIGGER IF EXISTS dnamebeforeupdate; DELIMITER // CREATE TRIGGER dnamebeforeupdate BEFORE UPDATE ON department FOR EACH ROW BEGIN SET NEW.dname = UPPER(NEW.dname); END// DELIMITER ; #An update statement that fires the trigger UPDATE department SET dname = 'Research Department 2' WHERE dnumber = 5; select * from department where dnumber = 5; 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'; 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. Example use company; SHOW VARIABLES LIKE 'event_scheduler'; select * from works_on_info; 4

5 drop event if exists work_on_info_update; show create table works_on_info; set global event_scheduler = on; #should be global DELIMITER // CREATE EVENT work_on_info_update ON SCHEDULE AT NOW() + INTERVAL 1 MONTH DO BEGIN delete from works_on_info; insert into works_on_info(emp_name, Proj_name, Hours_per_week) select E.Lname, P.Pname, W.Hours from project P, works_on W, employee E where P.pnumber=W.pno and W.essn=E.ssn; END // DELIMITER ; select * from works_on_info; Transaction Examples /** By default, MySQL runs in autocommit mode start transaction turns off autocommmit mode (set autocommit=0) until the statements in the transaction are commoitted or rolled back */ use company; delete from employee where ssn= ' '; select * from department; start transaction; savepoint before_insert_bob; savepoint before_update_bob; set sql_safe_updates = 0; update employee set salary= where ssn= ; 5

6 rollback to savepoint before_insert_bob; rollback to savepoint before_update_bob; commit; select count(*) from employee; select * from employee where ssn= ' '; update employee set salary= where ssn= ; delete from employee where ssn= ' '; drop procedure if exists transactiontest1; delimiter // create procedure transactiontest1() begin declare sqlerror tinyint default false; #sets sqlerror true if a transaction error occurs declare continue handler for sqlexception set sqlerror = true; start transaction; set sql_safe_updates = 0; update employee set salary= where ssn= ' '; if sqlerror = false then commit; select 'the transaction was committed'; else rollback; select 'the transaction was rolled back'; end if; end // delimiter ; call transactiontest1(); select * from employee where ssn= ' '; 6

7 delete from employee where ssn= ' '; drop procedure if exists transactiontest2; delimiter // create procedure transactiontest2() begin declare sqlerror tinyint default false; #sets sqlerror true if a transaction error occurs declare continue handler for sqlexception set sqlerror = true; start transaction; end// if sqlerror = false then commit; select 'the transaction was committed'; else rollback; select 'the transaction was rolled back'; end if; delimiter ; call transactiontest2(); select * from employee where ssn= ' '; delete from employee where ssn= ' '; drop procedure if exists transactiontest2; select count(*) from employee; 7