1. Advanced SQL: Views & Triggers
Instructor: Mohamed Eltabakh

2. Views

3. What is a View
An SQL query that we register (store) inside the database
Any query can be a view
CREATE VIEW <name> AS <select statement>;
DROP VIEW <name>;
ProfNumStudents is a view with schema (pNumber, CNT)
CREATE VIEW ProfNumStudents AS
SELECT pNumber, count(*) AS CNT
FROM Student
GROUP BY pNumber;

4. Why Need a View Frequent queries: query is used again and again
Complex queries: query written once and stored in the database
Logical data independence: the base table may change but the the view is still the same
Hide information (Security): allow users to see the view but not the original tables
CREATE VIEW StudentBasicInfo AS
SELECT sNumber, sName
FROM Student;
See only sNumber and sName

5. View Schema
You can think of a view as a table, but it gets its data during runtime
Only the definition is stored without data
View Schema
Consists of the columns produced from the select statement
CREATE VIEW ProfNumStudents AS
SELECT pNumber, count(*) AS CNT
FROM Student
GROUP BY pNumber;
ProfNumStudents(pNumber, CNT)
CREATE VIEW StudentBasicInfo AS
SELECT sNumber, sName
FROM Student;
StudentBasicInfo(sNumber, sName)

6. Example
Customers who have accounts
Customers who have loans

7. Example (Cont’d) , ‘A’ as type , ‘L’ as type
In this example, we added an extra column (constant) to differentiate between the two customer types
, ‘A’ as type
, ‘L’ as type

8. Querying a View
Exactly as querying a table

9. Triggers

10. Triggers: Introduction
The application constraints need to be captured inside the database
Some constraints can be captured by:
Primary Keys, Foreign Keys, Unique, Not NULL, and domain
Other application constraints are more complex
Need for assertions and triggers
Sum of loans taken by a customer does not exceed 100,000

11. Triggers
A procedure that runs automatically when a certain event occurs in the DBMS
The procedure performs some actions, e.g.,
Check certain values
Fill in some values
Inserts/deletes/updates other records
Check that some business constraints are satisfied
Commit (approve the transaction) or roll back (cancel the transaction)

12. Lets see how to define these components
Trigger Components
Three components
Event: When this event happens, the trigger is activated
Condition (optional): If the condition is true, the trigger executes, otherwise skipped
Action: The actions performed by the trigger
Semantics
When the Event occurs and Condition is true, execute the Action
Lets see how to define these components

13. Trigger: Event Example That is the event Create Trigger <name>
Before|After Insert|Update|Delete ON <tablename> ….
That is the event
Example
Create Trigger ABC
Before Insert On Students ….
This trigger is activated when an insert statement is issued, but before the new record is inserted
Create Trigger XYZ
After Update On Students ….
This trigger is activated when an update statement is issued and after the update is executed

14. Granularity of Event
An UPDATE or DELETE statement may update (or delete) many records at the same time
May insert many records in the same statement as well
Does the trigger execute for each updated or deleted record, or once for the entire statement ?
We define such granularity
Create Trigger <name>
Before| After Insert| Update| Delete
For Each Row | For Each Statement ….
That is the event
That is the granularity

15. Example: Granularity of Event
Create Trigger XYZ
After Update ON <tablename>
For each statement ….
This trigger is activated once (per UPDATE statement) after all records are updated
Create Trigger XYZ
Before Delete ON <tablename>
For each row ….
This trigger is activated before deleting each record

16. Trigger: Condition This component is optional That is the condition
Create Trigger <name>
Before| After Insert| Update| Delete On <tableName>
For Each Row | For Each Statement
When <condition> …
That is the condition
If the employee salary > 150,00, then some actions will be taken
Create Trigger EmpSal
After Insert or Update On Employee
For Each Row
When (New.salary >150,000) …

17. Use REFERENCING Clause
Trigger: Action
Action depends on what you want to do, e.g.:
Check certain values
Fill in some values
Inserts/deletes/updates other records
Check that some business constraints are satisfied
Commit (approve the transaction) or roll back (cancel the transaction)
In the action, you may want to reference:
The new values of inserted or updated records
The old values of deleted or updated records
Use REFERENCING Clause

18. Trigger: Referencing Clause
Create Trigger <name>
Before| After Insert| Update| Delete On <tableName>
Referencing
OLD AS oldRec,
NEW AS newRec
For Each Row | For Each Statement
When <condition>
Begin ….
End;
That is Referencing clause
That is the action
Now, can reference the old record as oldRec and new record as newRec
Insert event does not have old record
Delete event does not have new record
Update event has both old and new

19. Example 1
If the employee salary increased by more than 10%, make sure the ‘comment’ field is not empty and its value has changed, otherwise reject the update
Create Trigger EmpSal
Before Update On Employee
Referencing
OLD ROW AS oldRec,
NEW ROW AS newRec
For Each Row
Begin
IF (newRec.salary > oldRec.salary * 1.10) Then
IF (newRec.comment = ‘’ or newRec.comment is null or
newRec.comment = oldRec.comment)
RAISE_APPLICATION_ERROR(-20004, ‘Comment field not correct’);
End IF;
End;

20. Example 2: Using Temp Variable
If the newly inserted record in employee has null date field, fill it in with the current date
Create Trigger EmpDate
Before Insert On Employee
Referencing
NEW ROW AS newRec
For Each Row
Declare
temp date;
Begin
Select sysdate into temp from dual;
IF (newRec.date is null) Then
newRec.date := temp;
End IF;
End;
Define variables
Oracle system table always has the current date
Updating the new value to be inserted

21. Example 3: Maintenance of Derived Attributes
Keep the bonus attribute in Employee table always 3% of the salary attribute
Create Trigger EmpBonus
Before Insert Or Update On Employee
For Each Row
Begin
newRec.bonus := newRec.salary * 0.03;
End;
Indicate two events at the same time
The bonus value is always computed automatically