1. Lecture 06: SQL Systems Aspects
Friday, October 10, 2003

2. Outline Two Examples Views (6.7) -- finish up Constraints (Chapter 7)
Embedded SQL (8.1)
Transactions in SQL (8.6)

3. Two Examples Store(sid, sname) Product(pid, pname, price, sid)
Find all stores that sell only products with price > 100
(Equivalent formulation:
find all stores s.t. all their products have price > 100)

4. SELECT Store.name
FROM Store, Product
WHERE Store.sid = Product.sid
GROUP BY Store.sid
HAVING 100 < min(Product.price)
SELECT Store.name
FROM Store
WHERE < ALL (SELECT Product.price FROM product WHERE Store.sid = Product.sid)
Your pick…
SELECT Store.name
FROM Store
WHERE Store.sid NOT IN
(SELECT Product.sid
FROM Product
WHERE Product.price <= 100)

5. Two Examples Store(sid, sname) Product(pid, pname, price, sid)
For each store, find the Product ID of its most expensive product

6. Two Examples This is easy but doesn’t do what we want:
SELECT Store.name, max(Product.price)
FROM Store, Product
WHERE Store.sid = Product.sid
GROUP BY Store.sid
Better:
SELECT Store.name, x.pid
FROM Store, Product x
WHERE Store.sid = x.sid and
x.price >= ALL (SELECT y.price FROM Product y WHERE Store.sid = y.sid)
But may return multiple pids

7. Two Examples
Finally, choose some pid arbitrarily, if there are many with highest price:
SELECT Store.name, max(x.pid)
FROM Store, Product x
WHERE Store.sid = x.sid and
x.price >= ALL (SELECT y.price FROM Product y WHERE Store.sid = y.sid) GROUP BY Store.name

8. Updating Views
How can I insert a tuple into a table that doesn’t exist?
Employee(ssn, name, department, project, salary)
CREATE VIEW Developers AS
SELECT name, project
FROM Employee
WHERE department = “Development”
If we make the
following insertion:
INSERT INTO Developers VALUES(“Joe”, “Optimizer”)
INSERT INTO Employee(ssn, name, department, project, salary) VALUES(NULL, “Joe”, NULL, “Optimizer”, NULL)
It becomes:

9. Non-Updatable Views Person(name, city)
Purchase(buyer, seller, product, store)
CREATE VIEW City-Store AS
SELECT Person.city, Purchase.store
FROM Person, Purchase
WHERE Person.name = Purchase.buyer
How can we add the following tuple to the view?
(“Seattle”, “Nine West”)
We don’t know the name of the person who made the purchase; cannot set to NULL (why ?)

10. Constraints in SQL
A constraint = a property that we’d like our database to hold
The system will enforce the constraint by taking some actions:
forbid an update
or perform compensating updates

11. Constraints in SQL Constraints in SQL: Keys, foreign keys
Attribute-level constraints
Tuple-level constraints
Global constraints: assertions
The more complex the constraint, the harder it is to check and to enforce
simplest
Most complex

12. Keys OR: CREATE TABLE Product ( name CHAR(30) PRIMARY KEY,
category VARCHAR(20))
OR:
CREATE TABLE Product (
name CHAR(30),
category VARCHAR(20)
PRIMARY KEY (name))

13. Keys with Multiple Attributes
CREATE TABLE Product (
name CHAR(30),
category VARCHAR(20),
price INT,
PRIMARY KEY (name, category))
Name
Category
Price
Gizmo
Gadget 10
Camera
Photo 20 30 40

14. Other Keys CREATE TABLE Product ( productID CHAR(10), name CHAR(30),
category VARCHAR(20),
price INT, PRIMARY KEY (productID),
UNIQUE (name, category))
There is at most one PRIMARY KEY; there can be many UNIQUE

15. Foreign Key Constraints
Referential integrity constraints
CREATE TABLE Purchase (
prodName CHAR(30)
REFERENCES Product(name),
date DATETIME)
prodName is a foreign key to Product(name) name must be a key in Product

16. Product Purchase Name Category Gizmo gadget Camera Photo OneClick
ProdName
Store
Gizmo
Wiz
Camera
Ritz

17. Foreign Key Constraints
(name, category) must be a PRIMARY KEY
CREATE TABLE Purchase (
prodName CHAR(30),
category VARCHAR(20),
date DATETIME,
FOREIGN KEY (prodName, category)
REFERENCES Product(name, category)

18. What happens during updates ?
Types of updates:
In Purchase: insert/update
In Product: delete/update
Product
Purchase
Name
Category
Gizmo
gadget
Camera
Photo
OneClick
ProdName
Store
Gizmo
Wiz
Camera
Ritz

19. What happens during updates ?
SQL has three policies for maintaining referential integrity:
Reject violating modifications (default)
Cascade: after a delete/update do a delete/update
Set-null set foreign-key field to NULL
READING ASSIGNEMNT: 7.1.5, 7.1.6

20. Constraints on Attributes and Tuples
Constraints on attributes: NOT NULL -- obvious meaning... CHECK condition -- any condition !
Constraints on tuples CHECK condition

21. What is the difference from Foreign-Key ?
CREATE TABLE Purchase (
prodName CHAR(30)
CHECK (prodName IN SELECT Product.name FROM Product),
date DATETIME NOT NULL)

22. General Assertions CREATE ASSERTION myAssert CHECK NOT EXISTS(
SELECT Product.name FROM Product, Purchase WHERE Product.name = Purchase.prodName GROUP BY Product.name HAVING count(*) > 200)

23. Final Comments on Constraints
Can give them names, and alter later
Read in the book !!!
We need to understand exactly when they are checked
We need to understand exactly what actions are taken if they fail

24. Embedded SQL direct SQL (= ad-hoc SQL) is rarely used
in practice: SQL is embedded in some application code
SQL code is identified by special syntax

25. Impedance Mismatch Example: SQL in C:
C uses int, char[..], pointers, etc
SQL uses tables
Impedance mismatch = incompatible types

26. The Impedance Mismatch Problem
Why not use only one language?
Forgetting SQL: “we can quickly dispense with this idea” [textbook, pg. 351].
SQL cannot do everything that the host language can do.
Solution: use cursors

27. Programs with Embedded SQL
Host language + Embedded SQL
Preprocessor
Preprocessor
Call-level interface (CLI): ODBC,JDBC, ADO
Host Language + function calls
Host language compiler
Host language compiler
Host language program

28. Interface: SQL / Host Language
Values get passed through shared variables.
Colons precede shared variables when they occur within the
SQL statements.
EXEC SQL: precedes every SQL statement in the host language.
The variable SQLSTATE provides error messages and status
reports (e.g., “00000” says that the operation completed with no
problem).
EXEC SQL BEGIN DECLARE SECTION;
char productName[30];
EXEC SQL END DECLARE SECTION;

29. Example Product (pname, price, quantity, maker)
Purchase (buyer, seller, store, pname)
Company (cname, city)
Person(name, phone, city)

30. Using Shared Variables
Void simpleInsert() {
EXEC SQL BEGIN DECLARE SECTION;
char n[20], c[30]; /* product-name, company-name */
int p, q; /* price, quantity */
char SQLSTATE[6];
EXEC SQL END DECLARE SECTION;
/* get values for name, price and company somehow */
EXEC SQL INSERT INTO Product(pname, price, quantity, maker)
VALUES (:n, :p, :q, :c); }

31. Single-Row Select Statements
int getPrice(char *name) {
EXEC SQL BEGIN DECLARE SECTION;
char n[20];
int p;
char SQLSTATE[6];
EXEC SQL END DECLARE SECTION;
strcpy(n, name); /* copy name to local variable */
EXEC SQL SELECT price INTO :p
FROM Product
WHERE Product.name = :n;
return p; }

32. Cursors Declare the cursor Open the cursor Fetch tuples one by one
Close the cursor

33. Cursors void product2XML() { EXEC SQL BEGIN DECLARE SECTION;
char n[20], c[30];
int p, q;
char SQLSTATE[6];
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE crs CURSOR FOR
SELECT pname, price, quantity, maker
FROM Product;
EXEC SQL OPEN crs;

34. Cursors printf(“<allProducts>\n”); while (1) {
EXEC SQL FETCH FROM crs INTO :n, :p, :q, :c;
if (NO_MORE_TUPLES) break;
printf(“ <product>\n”);
printf(“ <name> %s </name>\n”, n);
printf(“ <price> %d </price>\n”, p);
printf(“ <quantity> %d </quantity>\n”, q);
printf(“ <maker> %s </maker>\n”, c);
printf(“ </product>\n”); }
EXECT SQL CLOSE crs;
printf(“</allProducts>\n”);

35. What is NO_MORE_TUPLES ?
#define NO_MORE_TUPLES !(strcmp(SQLSTATE,”02000”))

36. More on Cursors cursors can modify a relation as well as read it.
We can determine the order in which the cursor will get
tuples by the ORDER BY keyword in the SQL query.
Cursors can be protected against changes to the
underlying relations.
The cursor can be a scrolling one: can go forward, backward
+n, -n, Abs(n), Abs(-n).

37. Dynamic SQL So far the SQL statements were visible to the compiler
In dynamic SQL we have an arbitrary string that represents a SQL command
Two steps:
Prepare: compiles the string
Execute: executes the compiled string

38. Dynamic SQL Void someQuery() { EXEC SQL BEGIN DECLARE SECTION;
char *command=“UPDATE Product SET quantity=quantity+1 WHERE name=“gizmo”
EXEC SQL END DECLARE SECTION;
EXEC SQL PREPARE myquery FROM :command;
EXEC SQL EXECUTE myquery; }
myquery = a SQL variable, does not need to be prefixed by “:”

39. Transactions Address two issues: Access by multiple users
Remember the “client-server” architecture: one server with many clients
Protection against crashes

40. Multiple users: single statements
Client 1:
UPDATE Product SET Price = Price – WHERE pname = ‘Gizmo’ Client 2: UPDATE Product SET Price = Price*0.5 WHERE pname=‘Gizmo’
Two managers attempt to do a discount. Will it work ?

41. Multiple users: multiple statements
Client 1: INSERT INTO SmallProduct(name, price) SELECT pname, price FROM Product
WHERE price <= 0.99
DELETE Product WHERE price <=0.99 Client 2: SELECT count(*) FROM Product SELECT count(*) FROM SmallProduct
What’s wrong ?

42. Protection against crashes
Client 1:
INSERT INTO SmallProduct(name, price) SELECT pname, price FROM Product
WHERE price <= 0.99
DELETE Product WHERE price <=0.99
Crash !
What’s wrong ?

43. Transactions
Transaction = group of statements that must be executed atomically
Transaction properties: ACID
ATOMICITY = all or nothing
CONSISTENCY = leave database in consistent state
ISOLATION = as if it were the only transaction in the system
DURABILITY = store on disk !

44. Transactions in SQL In “ad-hoc” SQL: In “embedded” SQL:
Default: each statement = one transaction
In “embedded” SQL:
BEGIN TRANSACTION
[SQL statements]
COMMIT or ROLLBACK (=ABORT)

45. Transactions: Serializability
Serializability = the technical term for isolation
An execution is serial if it is completely before or completely after any other function’s execution
An execution is serializable if it equivalent to one that is serial
DBMS can offer serializability guarantees

46. Serializability Enforced with locks, like in Operating Systems !
But this is not enough:
User 1
LOCK A
[write A=1]
UNLOCK A
LOCK B
[write B=2]
UNLOCK B
User 2
LOCK A
[write A=3]
UNLOCK A
LOCK B
[write B=4]
UNLOCK B
The final values are A=3, B=2. If the two transactions were executed serially, then we would have either A=1,B=2, or A=3,B=4. This is not a serializable execution
time
What is wrong ?

47. Serializability Solution: two-phase locking
Lock everything at the beginning
Unlock everything at the end
Read locks: many simultaneous read locks allowed
Write locks: only one write lock allowed
Insert locks: one per table

48. Isolation Levels in SQL
“Dirty reads”
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
“Committed reads”
SET TRANSACTION ISOLATION LEVEL READ COMMITTED
“Repeatable reads”
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
Serializable transactions (default):
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
Reading assignment: chapter 8.6