1. Database Management Systems
Chapter 7
Database Integrity and Transactions

2. Programming Environment
DBMS
Tables
Create code
(1) Within the query system
(2) In forms and reports
(3) Hosted in external programs
Queries
(1)
If ( . . ) Then
SELECT . . .
Else . . .
UPDATE . . .
End If
External
Program
Forms &
Reports
(3)
C++
if (. . .) {
// embed SQL
SELECT … }
(2)
If (Click) Then
MsgBox . . .
End If

3. User-Defined Function
CREATE FUNCTION EstimateCosts
(ListPrice Currency, ItemCategory VarChar)
RETURNS Currency
BEGIN
IF (ItemCategory = ‘Clothing’) THEN
RETURN ListPrice * 0.5
ELSE
RETURN ListPrice * 0.75
END IF
END

4. Function to Perform Conditional Update
CREATE FUNCTION IncreaseSalary
(EmpID INTEGER, Amt CURRENCY)
RETURNS CURRENCY
BEGIN
IF (Amt > 50000) THEN
RETURN error flag
END
UPDATE Employee SET Salary = Salary + Amt
WHERE EmployeeID = EmpID;
RETURN Amt

5. Looking Up Data CREATE FUNCTION IncreaseSalary
(EmpID INTEGER, Amt CURRENCY)
RETURNS CURRENCY
DECLARE
CURRENCY MaxAmount;
BEGIN
SELECT MaxRaise INTO MaxAmount
FROM CompanyLimits
WHERE LimitName = ‘Raise’;
IF (Amt > 50000) THEN
RETURN error flag
END
UPDATE Employee SET Salary = Salary + Amt
WHERE EmployeeID = EmpID;
RETURN Amt;

6. Data Trigger Events INSERT DELETE UPDATE BEFORE AFTER
Oracle additions:
Tables ALTER, CREATE, DROP
User LOGOFF, LOGON
Database SERVERERROR, SHUTDOWN, STARTUP

7. Statement v. Row Triggers
UPDATE Employee
SET Salary = Salary
WHERE EmployeeID=442
OR EmployeeID=558
SQL
After Update
On table
Before Update
On table
Triggers for overall table
Update
Row 442
After Update
Row 442
… other rows
Before Update
Row 442
time
Triggers for each row

8. Data Trigger Example CREATE TRIGGER LogSalaryChanges
AFTER UPDATE OF Salary ON Employee
REFERENCING OLD ROW as oldrow
NEW ROW AS newrow
FOR EACH ROW
INSERT INTO SalaryChanges
(EmpID, ChangeDate, User, OldValue, NewValue)
VALUES
(newrow.EmployeeID, CURRENT_TIMESTAMP,
CURRENT_USER, oldrow.Salary, newrow.Salary);

9. Canceling Data Changes in Triggers
CREATE TRIGGER TestDeletePresident
BEFORE DELETE ON Employee
REFERENCING OLD ROW AS oldrow
FOR EACH ROW
WHEN (oldrow.Title = ‘President’)
SIGNAL _CANNOT_DELETE_PRES;

10. Cascading Triggers Sale(SaleID, SaleDate, …)
SaleItem(SaleID, ItemID, Quantity, …)
AFTER INSERT
UPDATE Inventory
SET QOH = QOH – newrow.Quantity
Inventory(ItemID, QOH, …)
AFTER UPDATE
WHEN newrow.QOH < newrow.Reorder
INSERT {new order}
INSERT {new OrderItem}
Order(OrderID, OrderDate, …)
OrderItem(OrderID, ItemID, Quantity, …)

11. Trigger Loop Employee(EID, Salary) AFTER UPDATE
IF newrow.Salary > THEN
Add Bonus
END
BonusPaid(EID, BonusDate, Amount)
AFTER UPDATE Or INSERT
IF newrow.Bonus > THEN
Reduce Bonus
Add Options
END
StockOptions(EID, OptionDate, Amount, SalaryAdj)
AFTER UPDATE Or INSERT
IF newrow.Amount > THEN
Reduce Salary
END

12. Transactions Some transactions result in multiple changes.
These changes must all be completed successfully, or the group must fail.
Protection for hardware and communication failures.
example: bank customer transfers money from savings account to checking account.
Decrease savings balance
Increase checking balance
Problem if one transaction and machine crashes.
Possibly: give users a chance to reverse/undo a transaction.
Performance gain by executing transactions as a block.
Savings Accounts
Inez:
$1000
Checking Accounts
Inez:
Transaction
1. Subtract $1000 from savings.
(machine crashes)
2. Add $1000 to Checking.
(money disappears)

13. Defining Transactions
The computer needs to be told which changes must be grouped into a transaction.
Turn on transaction processing.
Signify a transaction start.
Signify the end.
Success: save all changes
Failure: cancel all changes
Must be set in module code
Commit
Rollback

14. SQL Transaction Code
CREATE FUNCTION TransferMoney(Amount Currency, AccountFrom Number,
AccountTo Number) RETURNS NUMBER
curBalance Currency;
BEGIN
DECLARE HANDLER FOR SQLEXCEPTION
ROLLBACK;
Return -2; -- flag for completion error
END;
START TRANSACTION; -- optional
SELECT CurrentBalance INTO curBalance
FROM Accounts WHERE (AccountID = AccountFrom);
IF (curBalance < Amount) THEN
RETURN -1; -- flag for insufficient funds
END IF
UPDATE Accounts
SET CurrentBalance = CurrentBalance – Amount
WHERE AccountID = AccountFrom;
SET CurrentBalance = CurrentBalance + Amount
WHERE AccountID = AccountTo;
COMMIT;
RETURN 0; -- flag for success

15. SAVEPOINT SAVEPOINT StartOptional start commit Required elements
Risky steps
time
Partial
rollback
START TRANSACTION;
SELECT …
UPDATE …
SAVEPOINT StartOptional;
If error THEN
ROLLBACK TO SAVEPOINT StartOptional;
END IF
COMMIT;

16. Concurrent Access Concurrent Access Force sequential Two processes
Multiple users or processes changing the same data at the same time.
Final data will be wrong!
Force sequential
Locking
Delayed, batch updates
Two processes
Receive payment ($200)
Place new order ($150)
Initial balance $800
Result should be $ = $750
Interference result is either $600 or $950
Customers
Receive Payment
Place New Order
ID Balance
Jones $800
$600
$950
1) Read balance 800
2) Subtract pmt -200
4) Save new bal. 600
3) Read balance 800
5) Add order 150
6) Write balance 950

17. Pessimistic Locks: Serialization
One answer to concurrent access is to prevent it.
When a transaction needs to alter data, it places a SERIALIZABLE lock on the data used, so no other transactions can even read the data until the first transaction is completed.
SET TRANSACTION SERIALIZABLE, READ WRITE
Customers
Receive Payment
Place New Order
ID Balance
Jones $800
$600
1) Read balance 800
2) Subtract pmt -200
4) Save new bal. 600
3) Read balance
Receive error message that it is locked.

18. SQL Pessimistic Lock CREATE FUNCTION ReceivePayment (
AccountID NUMBER, Amount Currency) RETURNS NUMBER
BEGIN
DECLARE HANDLER FOR SQLEXCEPTION
ROLLBACK;
RETURN -2;
END
SET TRANSACTION SERIALIZABLE, READ WRITE;
UPDATE Accounts
SET AccountBalance = AccountBalance - Amount
WHERE AccountNumber = AccountID;
COMMIT;
RETURN 0;

19. Deadlock Deadlock Many solutions Data A Data B 1) Lock Data A
3) Wait for Data B
Deadlock
Two (or more) processes have placed locks on data and are waiting for the other’s data.
Many solutions
Random wait time
Global lock manager
Two-phase commit - messages
Data A
Data B
2) Lock Data B
4) Wait for Data A

20. Lock Manager

21. Optimistic Locks Assume that collisions are rare
Improved performance, fewer resources
Allow all code to read any data (no locks)
When code tries to write a new value
Check to see if the existing value is different from the one you were given earlier
If it is different, someone changed the database before you finished, so it is a collision--raise an error
Reread the value and try again

22. Optimistic Locks for Simple Update
(1) Read the balance
(2) Add the new order value
(3) Write the new balance
(4) Check for errors
(5) If there are errors, go back to step (1).

23. Optimistic Locks with SQL
CREATE FUNCTION ReceivePayment (
AccountID NUMBER, Amount Currency) RETURNS NUMBER
oldAmount Currency;
testEnd Boolean = FALSE;
BEGIN
DO UNTIL testEnd = TRUE
SELECT Amount INTO oldAmount
WHERE AccountNumber = AccountID; …
UPDATE Accounts
SET AccountBalance = AccountBalance - Amount
WHERE AccountNumber = AccountID
AND Amount = oldAmount;
COMMIT;
IF SQLCODE = 0 and nrows > 0 THEN
testEnd = TRUE;
RETURN 0;
END IF
-- keep a counter to avoid infinite loops
END

24. ACID Transactions Atomicity: all changes succeed or fail together.
Consistency: all data remain internally consistent (when committed) and can be validated by application checks.
Isolation: The system gives each transaction the perception that it is running in isolation. There are no concurrent access issues.
Durability: When a transaction is committed, all changes are permanently saved even if there is a hardware or system failure.

25. SQL 99/200x Isolation Levels
READ UNCOMMITTED
Problem: might read dirty data that is rolled back
Restriction: not allowed to save any data
READ COMMITTED
Problem: Second transaction might change or delete data
Restriction: Need optimistic concurrency handling
REPEATABLE READ
Problem: Phantom rows
SERIALIZABLE
Provides same level of control as if all transactions were run sequentially.
But, still might encounter locks and deadlocks

26. Phantom Rows ItemID QOH Price 111 5 15 113 6 7 117 12 30 118 4 119 22
SELECT SUM(QOH)
FROM Inventory
WHERE Price BETWEEN 10 and 20
ItemID
QOH
Price
111 5 15
113 6 7
117 12 30
118 4
119 22
120 8 17
Included in first query
UPDATE Inventory
SET Price = Price/2
WHERE …
Additional rows will be included in the second query
SELECT SUM(QOH)
FROM Inventory
WHERE Price BETWEEN 10 and 20

27. Generated Keys Create an order for a new customer:
Customer Table
CustomerID, Name, …
Create an order for a new customer:
(1) Create new key for CustomerID
(2) INSERT row into Customer
(3) Create key for new OrderID
(4) INSERT row into Order
Order Table
OrderID, CustomerID, …

28. Methods to Generate Keys
The DBMS generates key values automatically whenever a row is inserted into a table.
Drawback: it is tricky to get the generated value to use it in a second table.
A separate key generator is called by a programmer to create a new key for a specified table.
Drawback: programmers have to write code to generate a key for every table and each row insertion.
Overall drawbacks: neither method is likely to be transportable. If you change the DBMS, you will have to rewrite the procedures to generate keys.

29. Auto-Generated Keys Create an order for a new customer:
INSERT row into Customer
Get the key value that was generated
Verify the key value is correct. How?
INSERT row into Order
Major problem:
Step 2 requires that the DBMS return the key value that was most recently generated. How do you know it is the right value? What happens if two transactions generate keys at almost the same time on the same table?

30. Key-Generation Routine
Create an order for a new customer:
Generate a key for CustomerID
INSERT row into Customer
Generate a key for OrderID
INSERT row into Order
This method ensures that unique keys are generated, and that you can use the keys in multiple tables because you know the value. But, none of it is automatic. It always requires procedures and sometimes data triggers.

31. Database Cursors Purpose Why?
Track through table or query one row at a time.
Data cursor is a pointer to active row.
Why?
Performance.
SQL cannot do everything.
Complex calculations.
Compare multiple rows.
Year Sales
,321
,998
,004
,736
,321
,998
,004
,736

32. Database Cursor Program Structure
DECLARE cursor1 CURSOR FOR
SELECT AccountBalance
FROM Customer;
sumAccount, balance Currency;
SQLSTATE Char(5);
BEGIN
sumAccount = 0;
OPEN cursor1;
WHILE (SQLSTATE = ‘00000’)
FETCH cursor1 INTO balance;
IF (SQLSTATE = ‘00000’) THEN
sumAccount = sumAccount + balance;
END IF
END
CLOSE cursor1;
-- display the sumAccount or do a calculation

33. Cursor Positioning with FETCH
DECLARE cursor2 SCROLL CURSOR FOR
SELECT …
OPEN cursor2;
FETCH LAST FROM cursor2 INTO …
Loop…
FETCH PRIOR FROM cursor2 INTO …
End loop
CLOSE cursor2;
FETCH positioning options:
FETCH NEXT next row
FETCH PRIOR prior row
FETCH FIRST first row
FETCH LAST last row
FETCH ABSOLUTE 5 fifth row
FETCH RELATIVE -3 back 3 rows

34. Problems with Multiple Users
Original Data
Modified Data
Name Sales
Alice 444,321
Carl 254,998
Donna 652,004
Ed 411,736
Name Sales
Alice 444,321
Bob 333,229
Carl 254,998
Donna 652,004
Ed 411,736
New row is
added--while
code is running.
The SQL standard can prevent this problem with the INSENSITIVE option:
DECLARE cursor3 INSENSITIVE CURSOR FOR …
But, this is an expensive approach, because the DBMS usually makes a copy of the data. Instead, avoid moving backwards.

35. Changing Data with Cursors
DECLARE cursor1 CURSOR FOR
SELECT Year, Sales, Gain
FROM SalesTotal
ORDER BY Year
FOR UPDATE OF Gain;
priorSales, curYear, curSales, curGain
BEGIN
priorSales = 0;
OPEN cursor1;
Loop:
FETCH cursor1 INTO curYear, curSales, curGain
UPDATE SalesTotal
SET Gain = Sales – priorSales
WHERE CURRENT OF cursor1;
priorSales = curSales;
Until end of rows
CLOSE cursor1;
COMMIT;
END
Year
Sales
Gain
2000
151,039
2001
179,332
2002
195,453
2003
221,883
2004
223,748

36. Dynamic Parameterized Cursor Queries
DECLARE cursor2 CURSOR FOR
SELECT ItemID, Description, Price
FROM Inventory
WHERE Price < :maxPrice;
maxPrice Currency;
BEGIN
maxPrice = … -- from user or other query
OPEN cursor2; -- runs query with current value
Loop:
-- Do something with the rows retrieved
Until end of rows
CLOSE cursor2;
END
Parameters enable you to control the rows retrieved dynamically from within the procedure code. The value is applied when the cursor is opened.

37. Sally’s Pet Store Inventory
Inventory method 1: calculate the current quantity on hand by totaling all purchases and sales every time the total is needed.
Drawback: performance
Inventory method 2: keep a running balance in the inventory table and update it when an item is purchased or sold.
Drawback: tricky code
Also, you need an adjustment process for “inventory shrink”

38. Inventory QuantityOnHand
Merchandise
ItemID
Description
QuantityOnHand
ListPrice
Category
Add items purchased
Subtract items sold
Adjust for shrink
SaleItem
SaleID
ItemID
Quantity
SalePrice

39. Inventory Events Add a row. Delete a row. Update Quantity.
For a new sale, a row is added to the SaleItem table.
A sale or an item could be removed because of a clerical error or the customer changes his or her mind. A SaleItem row will be deleted.
An item could be returned, or the quantity could be adjusted because of a counting error. The Quantity is updated in the SaleItem table.
An item is entered incorrectly. ItemID is updated in the SaleItem table.
SaleItem
SaleID
ItemID
Quantity
SalePrice
Add a row.
Delete a row.
Update Quantity.
Update ItemID.

40. New Sale: Insert SaleItem Row
CREATE TRIGGER NewSaleItem
AFTER INSERT ON SaleItem
REFERENCING NEW ROW AS newrow
FOR EACH ROW
UPDATE Merchandise
SET QuantityOnHand = QuantityOnHand – newrow.Quantity
WHERE ItemID = newrow.ItemID;

41. Delete SaleItem Row CREATE TRIGGER DeleteSaleItem
AFTER DELETE ON SaleItem
REFERENCING OLD ROW AS oldrow
FOR EACH ROW
UPDATE Merchandise
SET QuantityOnHand = QuantityOnHand + oldrow.Quantity
WHERE ItemID = oldrow.ItemID;

42. Quantity Changed Event
CREATE TRIGGER UpdateSaleItem
AFTER UPDATE ON SaleItem
REFERENCING OLD ROW AS oldrow
NEW ROW AS newrow
FOR EACH ROW
UPDATE Merchandise
SET QuantityOnHand = QuantityOnHand
+ oldrow.Quantity – newrow.Quantity
WHERE ItemID = oldrow.ItemID;

43. ItemID or Quantity Changed Event
CREATE TRIGGER UpdateSaleItem
AFTER UPDATE ON SaleItem
REFERENCING OLD ROW AS oldrow
NEW ROW AS newrow
FOR EACH ROW
BEGIN
UPDATE Merchandise
SET QuantityOnHand = QuantityOnHand + oldRow.Quantity
WHERE ItemID = oldrow.ItemID;
SET QuantityOnHand = QuantityOnHand – newRow.Quantity
WHERE ItemID = newrow.ItemID;
COMMIT;
END