Database Automation with MySQL Triggers and Event Schedulers

S e m i n a r o n D a t a b a s e A u t o m a t i o n
w i t h
M y S Q L T r i g g e r s
a n d
E v e n t S c h e d u l e r s
A b d u l R a h m a n S h e r z a d
L e c t u r e r a t C o m p u t e r S c i e n c e f a c u l t y o f H e r a t U n i v e r s i t y , A f g h a n i s t a n
P h D S t u d e n t a t T U - B e r l i n , G e r m a n y
J a n u a r y 2 7 , 2 0 1 5

MySQL Triggers and Events
• MySQL Triggers are operations that are automatically performed when a
specified database event (INSERT, UPDATE, DELETE) occurs.
• Ideally, triggers should be considered when they automate changes which are
specific to the database or management of its data.
– Data integrity, logging and auditing, business logic, perform calculations, run
further SQL commands, etc.
• MySQL Events can be scheduled to run once or more during specific periods.
Effectively, they are database-only cron-jobs and can be used in many cases
such as:
– optimizing database tables, archiving data, cleaning up logs, or generate complex
reports during off-peak time.
2

Introduction to MySQL Triggers
• A trigger is a set of SQL statements that is invoked or fired automatically when a
change is made to the data on the associated table either before or after the data
is changed by DML statements (INSERT, UPDATE or DELETE).
• MySQL version 5.7.2 allows you to define maximum six triggers for each table.
– BEFORE INSERT
– AFTER INSERT
– BEFORE UPDATE
– AFTER UPDATE
– BEFORE DELETE
– AFTER DELETE
MySQL 5.7.2+ removes this limitation and allows creation of multiple triggers for
the same event and action time in a table.
4

Introduction to MySQL Triggers II
• The trigger is not invoked with statement that makes change to the table but does
not use INSERT, DELETE or UPDATE statement.
– For example, the TRUNCATE statement removes the whole data of a table but does not
invoke the trigger associated with that table.
• The triggers are invoked with statements that use the INSERT statement behind
the scenes.
– For example, REPLACE statement and LOAD DATA statement.
• Database triggers are special types of stored procedures.
– Special because they are not called explicitly like stored procedures.
• Triggers are supported in MySQL, PostgreSQL, SQLite, Firebird, DB2, Microsoft
SQL Server and Oracle.
– Implementation varies, therefore, the appropriate documentation before coding should
be checked.
5

MySQL Triggers
Advantages
• SQL triggers provide an alternative
way to check the integrity of data.
• SQL triggers provide an alternative
way to run scheduled tasks.
– By using SQL triggers, no need to
wait to run the scheduled tasks
because the triggers are
invoked automatically before or
after a change is made to the data in
tables.
• SQL triggers are very useful to log
and audit the changes of data in
tables.
Disadvantages
• SQL triggers only can provide an extended
validation and they cannot replace all the
validations. Some simple validations have to be
done in the application layer. For example, user's
inputs can be validated on the client side or server
side.
• SQL triggers are invoked and executed invisibly
from client-applications therefore it is difficult to
figure out what happen in the database layer.
• SQL triggers may increase the overhead of the
database server.
6

Caution!!!
• Triggers should not be used in place of foreign key constraints.
• Triggers are not a substitute for transactions.
• Avoid duplication of effort.
– The back-end code should be sanitizing user input so it should not be necessary
to repeat those checks within the database.
• Triggers will execute quicker than a series of SQL commands passed by
back-end code
– but using complex triggers on regularly modified tables should be avoided.
7

MySQL Trigger Limitation
• MySQL triggers cover all features defined in the standard SQL. However,
triggers cannot use:
– SHOW, LOAD DATA, LOAD TABLE, BACKUP DATABASE, RESTORE, FLUSH and
RETURN statements.
– Statements that commit or rollback implicitly or explicitly such as COMMIT,
ROLLBACK, START TRANSACTION, LOCK / UNLOCK TABLES, ALTER, CREATE, DROP,
RENAME, etc.
– Prepared statements such as PREPARE, EXECUTE, etc.
– Dynamic SQL statements.
MySQL version 5.1.4+ a trigger can call a stored procedure or stored function, which
was a limitation is the previous versions.
8

MySQL Trigger Storage and Backup
• MySQL stores triggers in a data directory e.g.
/data/database_name/ with the files named table_name.TRG
and trigger_name.TRN
– The table_name.TRG file maps the trigger to the corresponding table.
– the trigger_name.TRN file contains the trigger definition.
• To back up the MySQL triggers
– Copy the trigger files to the backup directory.
– Use the mysqldump tool.
9

Create MySQL Trigger Basic Syntax
10
CREATE TRIGGER trigger_name trigger_time trigger_event
ON table_name
FOR EACH ROW
BEGIN
-- the trigger body
-- this code is applied to every
-- inserted/updated/deleted row
END;

Create MySQL Trigger Syntax Details
• The trigger name comes after the CREATE TRIGGER statement.
– The trigger name should follow the naming convention [table name]_[trigger
time]_ [trigger event].
• Trigger activation time can be BEFORE or AFTER.
• The trigger event can be INSERT, UPDATE or DELETE.
• The table name must specify after the ON keyword.
– A trigger must be associated with a specific table. Without a table trigger would not
exist.
• The trigger body is a set of SQL commands to run and are placed between
BEGIN and END block. This is where the logic for the trigger is defined.
11

Notice!
• In the trigger defined for INSERT
– NEW keyword can be used only.
– OLD keyword can not be used.
• In the trigger defined for DELETE
– there is no new row. Therefore, the OLD keyword can be used only.
• In the UPDATE trigger
– OLD keyword refers to the row before it is updated.
– NEW keyword refers to the row after it is updated.
12

MySQL Trigger
Application
Scenario
13

MySQL Trigger Application Scenario
• A small example database with two tables as follow:
– Blog: stores a unique ID, the title, content, and a deleted flag.
– Audit: stores a basic set of historical changes with an ID, the blog ID, the old blog
title, the change type (NEW, EDIT or DELETE), and the date/time of that change.
• INSERT into the blog table
– A new entry into the audit table containing the blog ID, the title, and a type of 'NEW'
(or 'DELETE' if it was deleted immediately) should be added.
• UPDATE in the blog table
– A new entry into the audit table containing the blog ID, the title, and a type of 'EDIT'
(or 'DELETE' if the deleted flag is set) should be added.
The 'changetime' attribute will automatically be set to the current time.
14

Blog Schema
CREATE TABLE blog (
id INT NOT NULL AUTO_INCREMENT,
title VARCHAR(255),
content text,
deleted TINYINT(1) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (id),
KEY (deleted)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
16

Audit Schema
CREATE TABLE audit (
id INT PRIMARY KEY NOT NULL AUTO_INCREMENT,
blog_id INT NOT NULL,
blog_title VARCHAR(255) NOT NULL,
changetype ENUM('NEW','EDIT','DELETE') NOT NULL,
changetime timestamp DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
KEY (blog_id),
CONSTRAINT FOREIGN KEY (blog_id) REFERENCES blog (id)
ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
17

Notice: Delimiter
• Most of the time the trigger body requires a number of SQL
commands separated by a semi-colon (;).
• To create the full trigger code the delimiter must be changed
to something else. For example
– $$
– //
• Finally, after trigger code completion the delimiter back should
be set to a semi-colon.
18

Trigger: AFTER INSERT
DELIMITER $$
CREATE TRIGGER blog_after_insert AFTER INSERT
ON blog
FOR EACH ROW
BEGIN
IF NEW.deleted THEN
SET @changetype = 'DELETE';
ELSE
SET @changetype = 'NEW';
END IF;
INSERT INTO audit (blog_id, blog_title, changetype) VALUES (NEW.id, NEW.title, @changetype);
END; $$
DELIMITER ;
19

Trigger: AFTER UPDATE
DELIMITER $$
CREATE TRIGGER blog_after_update AFTER UPDATE
ON blog
FOR EACH ROW
BEGIN
IF NEW.deleted THEN
SET @changetype = 'DELETE';
ELSE
SET @changetype = 'EDIT';
END IF;
INSERT INTO audit (blog_id, blog_title, changetype) VALUES (NEW.id, OLD.title, @changetype);
END; $$
DELIMITER ;
20

Test the Triggers
• Let's see what happens when we insert a new post into our blog table:
INSERT INTO blog (title, content) VALUES ('First blog', 'The first blog content');
SELECT * FROM blog;
SELECT * FROM audit;
• Let's update our blog text:
UPDATE blog SET title = 'First blog Edited' WHERE id = 1;
• Finally, let's mark the post as deleted:
UPDATE blog SET deleted = 1 WHERE id = 1;
21

Introduction to MySQL Events
• MySQL events were added in MySQL 5.1.6 and offer an alternative to
scheduled tasks and cron jobs.
• Events can be used to create backups, delete out-of-date and old records,
aggregate data for reports, etc.
• Unlike standard triggers which execute given a certain condition, an event
is an object that is triggered by the passage of time and is sometimes
referred to as a 'temporal trigger'.
• MySQL events can be scheduled to run either once or at a recurring interval
– when the server traffic will be low.
23

Starting the Event Scheduler
• The MySQL event scheduler is a process that runs in the
background and constantly looks for events to execute.
• Before creating or scheduling an event, first the scheduler
needs to turn on, with issuing the following command:
SET GLOBAL event_scheduler = ON;
• With the following the scheduler status is revealed if it is ON or
OFF:
SHOW PROCESSLIST;
24

Create MySQL Event Basic Syntax
CREATE EVENT event_name
ON SCHEDULE the_schedule
[ON COMPLETION [NOT] PRESERVE]
[ENABLE | DISABLE | DISABLE ON SLAVE]
DO BEGIN
-- the event body
END;
25

The Schedule Settings
• Run once on a specific date/time:
– AT 'YYYY-MM-DD HH:MM.SS'
• AT '2016-01-15 02:00.00'
• Run once after a specific period has elapsed:
– AT CURRENT_TIMESTAMP + INTERVAL n [HOUR|MONTH|WEEK|DAY|MINUTE]
• AT CURRENT_TIMESTAMP + INTERVAL 1 DAY
• Run at specific intervals forever:
– EVERY n [HOUR|MONTH|WEEK|DAY|MINUTE]
• EVERY 1 DAY
• Run at specific intervals during a specific period:
– EVERY n [HOUR|MONTH|WEEK|DAY|MINUTE] STARTS date ENDS date
• EVERY 1 DAY STARTS CURRENT_TIMESTAMP + INTERVAL 1 WEEK ENDS '2017-01-01 00:00.00'
26

Caution!!!
• An event is normally dropped once its schedule has expired
– ON COMPLETION NOT PRESERVE
• To prevent that behavior set
– ON COMPLETION PRESERVE
27

Run Once Event Example
DELIMITER $$
CREATE EVENT one_time_event_example
ON SCHEDULE AT CURRENT_TIMESTAMP + INTERVAL 5 MINUTE
DO BEGIN
UPDATE blog SET title = "One time event example" WHERE id = 1;
END; $$
DELIMITER ;
• This event will run once, one hour from the time it was created.
28

Preserved Event Example
DELIMITER $$
CREATE EVENT preserved_event_example
ON SCHEDULE AT CURRENT_TIMESTAMP + INTERVAL 1 HOUR
ON COMPLETION PRESERVE
DO BEGIN
UPDATE blog SET title = "Preserve event example" WHERE id = 1;
END; $$
DELIMITER ;
• To specify a preserved event, the ON COMPLETION PRESERVE clause
must be used.
29

Reoccurring Event Example
DELIMITER $$
CREATE EVENT reoccurring_event_example
ON SCHEDULE EVERY 1 HOUR
DO BEGIN
UPDATE blog SET title = "Reoccurring event example" WHERE id = 1;
END; $$
DELIMITER ;
• To specify a recurring event, the EVERY clause must be used.
• In this example, the reoccurring event would start tomorrow and continue
to run every hour for a full year.
30

MySQL Event
Application
Scenario
31

MySQL Event Application Scenario
• The database example used for trigger application has a problem.
– The old blog posts are marked as deleted rather than being removed from the blog
table.
– The blog table will grow indefinitely and become slower over time.
– The old blog posts could be purged, but that would remove them forever.
• As a result, we will move the blog posts and their associated audit records to
archive tables.
– The archive tables can grow without affecting the speed of the main web application and
we can undelete the old blog posts if necessary.
• Two archive tables are required:
– blog_archive: identical to the blog table except it does not require a deleted flag or an
auto-incrementing ID.
– audit_archive: identical to the audit table except the timestamp is not automatically
generated and it does not require an auto-incrementing ID.
32

MySQL Event Application Scenario II
• A scheduled event is required to:
– Copies blog posts from blog to blog_archive when the deleted flag is set
to 1.
– Copies the associated audit entries for those blog posts from audit to
audit_archive.
– Physically deletes the archived blog posts from the blog table.
– Referential integrity has been defined with a foreign key. Therefore, all
the associated audit entries for those blog posts will also be removed.
33

Blog Archive Schema
CREATE TABLE blog_archive (
id INT NOT NULL,
title VARCHAR(255),
content text,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
35

Audit Archive Schema
CREATE TABLE audit_archive (
id INT PRIMARY KEY NOT NULL,
blog_id INT NOT NULL,
blog_title VARCHAR(255),
changetype ENUM('NEW','EDIT','DELETE') NOT NULL,
changetime timestamp NOT NULL,
KEY (blog_id),
CONSTRAINT FOREIGN KEY (blog_id) REFERENCES blog_archive (id)
ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
36

The Archive Event
DELIMITER $$
CREATE EVENT archive_blog_posts
ON SCHEDULE EVERY 1 WEEK STARTS '2016-01-22 01:00:00'
DO BEGIN
-- copy deleted posts
INSERT INTO blog_archive (id, title, content) SELECT id, title, content FROM blog
WHERE deleted = 1;
-- copy associated audit records
INSERT INTO audit_archive (id, blog_id, blog_title, changetype, changetime)
SELECT audit.id, audit.blog_id, audit.blog_title, audit.changetype, audit.changetime
FROM audit JOIN blog ON audit.blog_id = blog.id WHERE blog.deleted = 1;
-- remove deleted blogs and audit entries
DELETE FROM blog WHERE deleted = 1;
END; $$
DELIMITER ;
• In the example, the event runs every week on Friday morning.
37

Wrap up
• The Triggers are powerful tools for protecting the integrity of
the data in the databases
– Data integrity, logging and auditing, business logic, perform
calculations, run further SQL commands, etc.
• The Events are very useful to automate some database
operations
– optimizing database tables, cleaning up logs, archiving data, or
generate complex reports during off-peak time.
38

Abdul Rahman
Sherzad
• Thank you for your participation
– Herat University
– Hariwa Institute of Higher Education
– Ghalib University
– Khaja Abdullah Ansari Institute of
Higher Education
– ICT Companies
Lecturer at Computer Science
faculty of Herat University,
Afghanistan.
PhD Student at Technical
University of Berlin (TU-Berlin),
Germany.
January 27, 2015
39

Database Automation with MySQL Triggers and Event Schedulers

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