Download SQL Triggers

Introduction to
Database Systems,
CS420
SQL Triggers
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Agenda
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Event-Condition-Action Rules
Trigger Definition
Options
Example
REVIEW: Attribute-Based Check
CREATE TABLE Sells (
bar
CHAR(20),
beer
CHAR(20) CHECK ( beer IN
(SELECT name FROM Beers)),
price REAL CHECK ( price <= 5.00 )
);
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REVIEW: Tuple-Based Check
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Only Joe’s Bar can sell beer for more than $5:
CREATE TABLE Sells (
bar
CHAR(20),
beer
CHAR(20),
price
REAL,
CHECK (bar = ’Joe’’s Bar’ OR
price <= 5.00)
);
Timing of Checks

Attribute-based checks are performed
only when a value for that attribute is
inserted or updated.
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Example: CHECK (price <= 5.00)
checks every new price and rejects the
modification (for that tuple) if the price is
more than $5.
Triggers
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Triggers are only executed when a
specified condition occurs, e.g., insertion
of a tuple.
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Easier to implement than complex
constraints.
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Triggers: Motivation
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Attribute- and tuple-based checks are
checked at known times, but are not
powerful.

Triggers let the user decide when to
check for any condition.
Usage of Trigger
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Enforce referential integrity across nodes
in a distributed database
Automatically generate derived column
values
Enforce complex business rules
Provide transparent event logging
Gather statistics on table access
Etc
Event-Condition-Action Rules
Another name for “trigger” is ECA rule, or
event-condition-action rule.
 Event : typically a type of database
modification, e.g., “insert on Sells.”
 Condition : Any SQL boolean-valued
expression.
 Action : Any SQL statements.
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Preliminary Example: A Trigger
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Instead of using a foreign-key constraint
and rejecting insertions into Sells(bar,
beer, price) with unknown beers, a
trigger can add that beer to Beers, with
a NULL manufacturer.
Example: Trigger Definition
The event
CREATE TRIGGER BeerTrig
AFTER INSERT ON Sells
REFERENCING NEW ROW AS NewTuple
FOR EACH ROW
The condition
WHEN (NewTuple.beer NOT IN
(SELECT name FROM Beers))
INSERT INTO Beers(name)
The action
VALUES(NewTuple.beer);
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Options: CREATE TRIGGER

CREATE TRIGGER <name>
Or:
CREATE OR REPLACE TRIGGER <name>
 Useful if there is a trigger with that name and
you want to modify the trigger.
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Options: The Event

AFTER can be BEFORE.
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INSERT can be DELETE or UPDATE.
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And UPDATE can be UPDATE . . . ON a
particular attribute.
Options: FOR EACH ROW
Triggers are either “row-level” or
“statement-level.”
 FOR EACH ROW indicates row-level; its
absence indicates statement-level.
 Row level triggers : execute once for each
modified tuple.
 Statement-level triggers : execute once for
a SQL statement, regardless of how many
tuples are modified.
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Options: REFERENCING

INSERT statements imply a new tuple (for
row-level) or new table (for statementlevel).

The “table” is the set of inserted tuples.
DELETE implies an old tuple or table.
 UPDATE implies both.
 Refer to these by
[NEW OLD][TUPLE TABLE] AS <name>
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Options: The Condition
Any boolean-valued condition.
 Evaluated on the database as it would
exist before or after the triggering event,
depending on whether BEFORE or
AFTER is used.
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But always before the changes take effect.
Access the new/old tuple/table through
the names in the REFERENCING
clause.
Options: The Action
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There can be more than one SQL
statement in the action.
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Surround by BEGIN . . . END if there is more
than one.
But queries make no sense in an action,
so we are really limited to modifications.
Example: Monitoring Logons
CREATE OR REPLACE TRIGGER
check_user
AFTER LOGON ON DATABASE
BEGIN
sec_mgr.check_user; executes the procedure
sec_mgr.check_user
END;
after a user logs onto
the database
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Another Example
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Using Sells(bar, beer, price) and a unary
relation RipoffBars(bar), maintain a list of
bars that raise the price of any beer by
more than $1.
The Trigger
The event –
only changes
to prices
CREATE TRIGGER PriceTrig
AFTER UPDATE OF price ON Sells
REFERENCING
Updates let us
talk about old
Condition:
OLD ROW AS ooo
and new tuples
a raise in
NEW ROW AS nnn We need to consider
price > $1
each price change
FOR EACH ROW
WHEN(nnn.price > ooo.price + 1.00)
INSERT INTO RipoffBars
When the price change
is great enough, add
VALUES(nnn.bar);
the bar to RipoffBars
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Example: Track Changes to
Emp_tab
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Create Demo Table
CREATE TABLE Emp_tab (
Empno
NUMBER NOT NULL,
Ename
VARCHAR2(10),
Job
VARCHAR2(9),
Mgr
NUMBER(4),
Hiredate DATE,
Sal
NUMBER(7,2),
Comm
NUMBER(7,2),
Deptno
NUMBER(2) NOT NULL);
)
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Insert Rows
INSERT INTO Emp_tab(101, ‘e1’, ‘SP’, 104, ‘10/20/2005’, 800, 7%, 10);
INSERT INTO Emp_tab(102, ‘e2’, ‘IT’, 201, ‘11/20/1990’, 900, 5%, 20);
INSERT INTO Emp_tab(103, ‘e3’, ‘IT’, 201, ‘07/20/1995’, 880, 5%, 20);
INSERT INTO Emp_tab(104, ‘e4’, ‘SP’, null, ‘01/20/2005’, 850, 6%, 10);
INSERT INTO Emp_tab(105, ‘e5’, ‘IT’, 201, ‘04/20/2010’, 950, 5%, 20);
INSERT INTO Emp_tab(201, ‘et’, ‘IT’, null, ‘12/20/1990’, 2000, 8%, 20);
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Display Data in the Table
SELECT
FROM
GROUP BY
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deptno, count(*)
Emp_tab
deptno;
Create Log Table

Set up the following data structure to track
salary increase
CREATE TABLE Emp_log (
Emp_id
NUMBER,
Log_date
DATE,
New_salary NUMBER,
Action
VARCHAR2(20));
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Create Trigger
CREATE OR REPLACE TRIGGER Log_salary_increase
AFTER UPDATE ON Emp_tab
REFERENCING NEW ROW as nnn
FOR EACH ROW
WHEN (nnn.Sal > 1000)
BEGIN
INSERT INTO Emp_log (Emp_id, Log_date,
New_salary, Action)
VALUES (nnn.Empno, SYSDATE, nnn.SAL, 'NEW SAL');
END;
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Run an Update
UPDATE
SET
WHERE
Emp_tab
Sal = Sal + 1000.0
Deptno = 20;
How many times will the
trigger be fired?
TRIGGER
Log_salary_increase
AFTER UPDATE ON Emp_tab
REFERENCING NEW ROW
as nnn
FOR EACH ROW
WHEN (nnn.Sal > 1000)
BEGIN
….
Since there are 4 employees in department 20, the trigger
fires 4 times when this statement is entered, because 4
rows are affected
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Another Trigger
CREATE OR REPLACE TRIGGER
Log_emp_update
AFTER UPDATE ON Emp_tab
BEGIN
INSERT INTO Emp_log (Log_date, Action)
VALUES (SYSDATE,
'Emp_tab COMMISSIONS CHANGED');
END;
fires only once
for each
UPDATE
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Clean Up
-- clean the table
drop table Emp_tab
Table dropped.
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Example: Enforce Security
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Set up Data Structure
CREATE TABLE Emp99 (
Empno
NOT NULL NUMBER(4),
Ename
VARCHAR2(10),
Job
VARCHAR2(9),
Mgr
NUMBER(4),
Hiredate
DATE,
Sal
NUMBER(7,2),
Comm
NUMBER(7,2),
Deptno
NUMBER(2),
Bonus
NUMBER,
Ssn
NUMBER,
Job_classification NUMBER);
CREATE TABLE Company_holidays (Day DATE);
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The Trigger
CREATE OR REPLACE TRIGGER
Emp_permit_changes
BEFORE INSERT OR DELETE OR UPDATE ON
Emp99
DECLARE
Dummy
INTEGER;
Not_on_weekends
EXCEPTION;
Not_on_holidays
EXCEPTION;
Non_working_hours
EXCEPTION;
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The Trigger (cont.)
BEGIN
/* Check for weekends: */
IF (TO_CHAR(Sysdate, 'DY') = 'SAT' OR
TO_CHAR(Sysdate, 'DY') = 'SUN')
THEN
RAISE Not_on_weekends;
END IF;
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The Trigger (cont.)
/* Check for company holidays: */
SELECT COUNT(*) INTO Dummy
FROM Company_holidays
WHERE TRUNC(Day) = TRUNC(Sysdate);
-- Discard time parts of dates
IF dummy > 0 THEN
RAISE Not_on_holidays;
END IF;
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The Trigger (cont.)
/* Check for work hours (8am to 6pm): */
IF (TO_CHAR(Sysdate, 'HH24') < 8 OR
TO_CHAR(Sysdate, 'HH24') > 18)
THEN
RAISE Non_working_hours;
END IF;
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The Trigger (cont.)
EXCEPTION
WHEN Not_on_weekends THEN
Raise_application_error(-20324,'Might not change '
|| 'employee table during the weekend');
WHEN Not_on_holidays THEN
Raise_application_error(-20325,'Might not change '
|| 'employee table during a holiday');
WHEN Non_working_hours THEN
Raise_application_error(-20326,'Might not change '
|| 'emp table during nonworking hours');
END;
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Clean Up
-- clean the tables
drop table Emp99;
drop table Company_holidays;
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Constraints vs. Triggers
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Constraints: statements about the database that
are always true.
 A constraint applies to existing data in the table
and any statement that manipulates the table
Triggers: constrain what a transaction can do.
 A trigger does not apply to data loaded before
the definition of the trigger
 it is not known if all data in a table conforms to
the rules established by an associated trigger
Summary
Event-Condition-Action rules
 Create trigger
 Options
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Event
 Condition
 Action
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END
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