1. SQL: Structured Query Language
Chapter 5 (cont.)
Constraints
Triggers
The slides for this text are organized into chapters. This lecture covers Chapter 5.
This is one of the most important chapters in any discussion of database systems. Students must acquire a solid grasp of SQL. In particular, learning how to write queries in SQL is important, and comes only with practice.
The slides present the concepts through examples. The chapter contains several additional examples with in-depth explanations; assign these as additional readings. The exercises contain numerous further examples, and come with supporting online material. If you need additional time to cover this material, consider abbreviating the earlier discussion of algebra and calculus, and reinforcing the same concepts in the context of SQL.
Note that some new SQL:1999 features for the HAVING clause are covered in these slides. (This material is not covered in the 2nd edition.) Also, material on cursors and other programmatic aspects of SQL has been moved to Chapter 6, following the revisions in the 3rd edition.

2. Integrity Constraints
Constraint describes conditions that every legal instance of a relation must satisfy.
Inserts/deletes/updates that violate ICs are disallowed.
Can be used to :
ensure application semantics
e.g., sid is a key
prevent inconsistencies
e.g., sname has to be a string,
age must be < 200; 5

3. Types of Integrity Constraints
Fundamental:
Domain constraints,
Primary key constraints,
Foreign key constraints
Plus NOT NULL, UNIQUE, etc.
General:
Check constraints (often limited to only simple ones)
Table constraints or assertions (but not available in most systems) 5

4. Check or Table Constraints
CREATE TABLE Sailors
( sid INTEGER,
sname CHAR(10),
rating INTEGER,
age REAL,
PRIMARY KEY (sid),
CHECK ( rating >= 1
AND rating <= 10 ))
Can use queries to express constraint.
VERY LIMITED IN MOST DBMSs!!!! 8

5. Explicit Domain Constraints
CREATE DOMAIN values-of-ratings INTEGER
DEFAULT 1
CHECK ( VALUE >= 1 AND VALUE <= 10)
CREATE TABLE Sailors
( sid INTEGER,
sname CHAR(10),
rating values-of-ratings,
age REAL,
PRIMARY KEY (sid)) 8

6. More Powerful Table Constraints
Constraint that Interlake boats cannot be reserved:
CREATE TABLE Reserves
( sname CHAR(10),
bid INTEGER,
day DATE,
PRIMARY KEY (bid,day),
CONSTRAINT noInterlakeRes
CHECK (`Interlake’ <>
( SELECT B.bname
FROM Boats B
WHERE B.bid= bid)))
If condition evaluates to FALSE, update is rejected. 8

7. Table Constraints Associated with one table
Only needs to hold TRUE when table is non-empty. 8

8. More Table Constraint Examples
Number of boats plus number of sailors is < 100
CREATE TABLE Sailors
( sid INTEGER,
sname CHAR(10),
rating INTEGER,
age REAL,
PRIMARY KEY (sid),
CHECK
( (SELECT COUNT (S.sid) FROM Sailors S)
+ (SELECT COUNT (B.bid) FROM Boats B)
< 100 )
Symmetric constraint, yet associated with Sailors.
If Sailors is empty, the number of Boats tuples can be anything! 9

9. Assertions: Constraints over Multiple Relations
CREATE TABLE Sailors
( sid INTEGER,
sname CHAR(10),
rating INTEGER,
age REAL,
PRIMARY KEY (sid), )
ASSERTION not associated with either table.
CREATE ASSERTION smallClub
CHECK
( (SELECT COUNT (S.sid) FROM Sailors S)
+ (SELECT COUNT (B.bid) FROM Boats B) < 100 )
Number of boats
plus number of
sailors is < 100 9

10. Caution !!!
Assertions or Check constraints with subqueries are NOT supported in Oracle !!!

11. Triggers : Make Database Active
Trigger: A procedure that starts automatically if specified changes occur to the DBMS
Analog to a "daemon" that monitors a database for certain events to occur

12. Triggers : Make Database Active
Three parts:
Event (activates the trigger)
Condition (tests whether the triggers should run) [Optional]
Action (what happens if the trigger runs)
Semantics:
When an event occurs,
And this condition is satisfied,
Then the associated action is performed.

13. Triggers – Event,Condition,Action
Events could be :
BEFORE|AFTER INSERT|UPDATE|DELETE ON <tableName>
e.g.: BEFORE INSERT ON Professor
Condition is an SQL expression (An SQL query with a non-empty result means TRUE.)
Action can be many different choices :
SQL statements , body of PSM, and even DDL and transaction-oriented statements like “commit”.

14. Example Trigger Assume our DB has a relation schema :
Professor (pNum, pName, salary)
We want to write a trigger that :
Ensures that any new professor inserted
has salary >= 90,000;

15. Example Trigger CREATE TRIGGER minSalary BEFORE INSERT ON Professor
for what context ?
BEGIN
check for violation here ?
END;

16. Example Trigger CREATE TRIGGER minSalary BEFORE INSERT ON Professor
FOR EACH ROW
BEGIN
Violation of Minimum Professor Salary?
END;

17. Example Trigger CREATE TRIGGER minSalary BEFORE INSERT ON Professor
FOR EACH ROW
BEGIN
IF (:new.salary < 90000)
THEN RAISE_APPLICATION_ERROR
(-20004,
‘Violation of Min Prof Salary’);
END IF;
END;

18. Example trigger CREATE TRIGGER minSalary BEFORE INSERT ON Professor
FOR EACH ROW
DECLARE temp int; -- dummy variable
BEGIN
IF (:new.salary < 90000)
THEN RAISE_APPLICATION_ERROR (-20004, ‘Violation of Minimum Professor Salary’);
END IF;
temp := 10; -- variable
END; .
Note: In SQLPLUS, don’t forget the “dot” after the trigger.

19. Details of Trigger Example
BEFORE INSERT ON Professor
This trigger is checked before tuple is inserted
FOR EACH ROW
specifies trigger is performed for each row inserted
:new
refers to new tuple inserted
If (:new.salary < 90000)
then an application error is raised and hence the row is not inserted; otherwise row is inserted.
Use error code: ;
Indicates value is in an invalid range

20. Example Trigger Using Condition CREATE TRIGGER minSalary BEFORE INSERT ON Professor FOR EACH ROW WHEN (new.salary < 90000) BEGIN RAISE_APPLICATION_ERROR (-20004, ‘Violation of Minimum Professor Salary’); END;
Conditions can refer to old/new values of tuples modified by the statement activating the trigger.

21. Triggers: REFERENCING
CREATE TRIGGER minSalary BEFORE INSERT ON Professor
REFERENCING NEW as newTuple
FOR EACH ROW
WHEN (newTuple.salary < 90000)
BEGIN
RAISE_APPLICATION_ERROR (-20004, ‘Violation of Minimum Professor Salary’);
END;

22. Example Trigger CREATE TRIGGER minSalary BEFORE UPDATE ON Professor
REFERENCING OLD AS oldTuple NEW as newTuple
FOR EACH ROW
WHEN (newTuple.salary < oldTuple.salary)
BEGIN
RAISE_APPLICATION_ERROR (-20004, ‘Salary Decreasing !!’);
END;
Ensure that salary does not decrease

23. Another Useful Example Trigger
CREATE TRIGGER minSalary
AFTER UPDATE on avail-parts ON INVENTORY
FOR EACH ROW
WHEN (new.avail-parts < new.reorderThres)
DECLARE x NUMBER;
BEGIN
SELECT COUNT(*) INTO x
FROM Pending_orders WHERE Part_no = :new.Part_no;
IF x = 0 THEN
INSERT INTO Pending_orders
VALUES (:new.Part_no,:new.Reorder_quant, sysdate);
END IF;
END;

24. Row vs Statement Level Trigger
Row level: activated once per modified tuple
Statement level: activate once per SQL statement
Row level triggers can access new data,
statement level triggers cannot generally do that (depends on DBMS).
Statement level triggers more efficient if we do not need to make row-specific decisions

25. Row vs Statement Level Trigger
Example: Consider a relation schema :
Account (num, amount)
where we will allow creation of new accounts only during normal business hours.

26. Example: Statement level trigger
CREATE TRIGGER MYTRIG1 BEFORE INSERT ON Account
FOR EACH STATEMENT
BEGIN
IF (TO_CHAR(SYSDATE,’dy’) IN (‘sat’,’sun’)) OR
(TO_CHAR(SYSDATE,’hh24:mi’) NOT BETWEEN ’08:00’ AND ’17:00’)
THEN
RAISE_APPLICATION_ERROR(-20500, ’Cannot create new account now !!’);
END IF;
END;
Note: “FOR EACH STATEMENT” is default; so you omit this clause in most systems.

27. Another Trigger Example (SQL:99)
CREATE TRIGGER youngSailorUpdate
AFTER INSERT ON SAILORS
REFERENCING NEW TABLE AS NewSailors
FOR EACH STATEMENT
INSERT
INTO YoungSailors(sid, name, age, rating)
SELECT sid, name, age, rating
FROM NewSailors N
WHERE N.age <= 18
Note: Oracle uses “ FOR EACH STATEMENT” as default, hence you would simply not list this. They also do not support reference to such a delta-table as above.

28. When to use BEFORE/AFTER
Based on semantics and on efficiency considerations.
Suppose we perform statement-level after insert, then all the rows are inserted first, then if the condition fails, and all the inserted rows must be “rolled back”
Not very efficient !

29. Combining multiple events into one trigger
CREATE TRIGGER salaryRestrictions
AFTER INSERT OR UPDATE ON Professor
FOR EACH ROW
BEGIN
IF (INSERTING AND :new.salary < 90000) THEN RAISE_APPLICATION_ERROR (-20004, 'below min salary'); END IF;
IF (UPDATING AND :new.salary < :old.salary) THEN RAISE_APPLICATION_ERROR (-20004, ‘Salary Decreasing !!'); END IF;
END;

30. Summary : Trigger Syntax
CREATE TRIGGER <triggerName>
BEFORE|AFTER INSERT|DELETE|UPDATE
[OF <columnList>] ON <tableName>|<viewName>
[REFERENCING [OLD AS <oldName>] [NEW AS <newName>]]
[FOR EACH ROW] (default is “FOR EACH STATEMENT”)
[WHEN (<condition>)]
<PSM body>;

31. Points about Triggers Check the system tables : user_triggers
user_trigger_cols
SELECT Trigger_type, Triggering_event, Table_name
FROM USER_TRIGGERS
WHERE Trigger_name = 'REORDER';
TYPE TRIGGERING_STATEMENT TABLE_NAME
AFTER EACH ROW UPDATE INVENTORY

32. Points about Triggers ORA-04091: mutating relation problem
In a row level trigger, you cannot have the body refer to the table specified in the event as it would see inconsistent data
Also INSTEAD OF triggers can be specified on views to model updateable views

33. Triggers and Oracle
Note 1: Running Triggers End CREATE TRIGGER statement with a dot and run:
Type the body of the trigger
Press '.' to terminate the sql statement
Type 'run' to create the trigger
Example:
> CREATE TRIGGER minSalary BEFORE ….
>…. END;
>.
> run;
Running the CREATE TRIGGER statement only creates the trigger; it does not execute it.
Only a triggering event, such as an insertion, causes the trigger to execute.

34. Triggers and Oracle Note : Displaying Trigger Definition Errors
If you get compilation errors, you can see the error
messages by typing in:
show errors trigger <trigger_name>;
Example:
> show errors trigger minSalary;
However , reported line numbers where
errors occur are not always useful.

35. Constraints versus Triggers
Constraints are useful for database consistency
Use IC when sufficient
More opportunity for optimization
Not restricted to insert/delete/update
Triggers are flexible and more powerful
Alerters
Event logging for auditing
Security enforcement
Analysis of table accesses (statistics)
Workflow and business logic
But can be hard to understand ……
Several triggers (Arbitrary order  unpredictable !?)
Chain triggers (When to stop ?)
Recursive triggers (Termination?)

36. Summary
SQL allows specification of rich integrity constraints and their efficient maintenance
Triggers respond to changes in the database: powerful for enforcing application semantics