Lecture 07: SQL Constraints and Programming

1. Lecture 07: SQLConstraints and Programming Wednesday, January 22, 2003

2. Agenda • Tales from real database research life • Constraints in SQL • Systems aspects of SQL.

3. 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

4. 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 Mostcomplex

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

6. CREATE TABLE Product ( name CHAR(30), category VARCHAR(20), price INT, PRIMARY KEY (name, category)) Keys with Multiple Attributes

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

8. Foreign Key Constraints Referentialintegrityconstraints 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

9. Product Purchase

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

11. What happens during updates ? Types of updates: • In Purchase: insert/update • In Product: delete/update Product Purchase

12. 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

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

14. Whatis the difference fromForeign-Key ? CREATE TABLE Purchase ( prodName CHAR(30) CHECK (prodName INSELECT Product.nameFROM Product), date DATETIME NOT NULL)

15. General Assertions CREATE ASSERTION myAssert CHECKNOT EXISTS( SELECT Product.nameFROM Product, PurchaseWHERE Product.name = Purchase.prodNameGROUPBY Product.nameHAVING count(*) > 200)

16. 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

17. Triggers in SQL • A trigger contains an event, a condition, an action. • Event = INSERT, DELETE, UPDATE • Condition = any WHERE condition (may refer to the old and the new values) • Action = more inserts, deletes, updates • Many, many more bells and whistles... • Read in the book (it only scratches the surface...)

18. 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

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

20. 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

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

22. 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;

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

24. 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); }

25. 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; }

26. Cursors • Declare the cursor • Open the cursor • Fetch tuples one by one • Close the cursor

27. 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;

28. 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”); }

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

30. 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).

31. 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

32. 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 “:”

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

34. Multiple users: single statements Client 1: UPDATE ProductSET Price = Price – 1.99WHERE pname = ‘Gizmo’Client 2:UPDATE ProductSET Price = Price*0.5WHERE pname=‘Gizmo’ Two managers attempt to do a discount.Will it work ?

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

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

37. 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 !

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

39. 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

40. Serializability • Enforced with locks, like in Operating Systems ! • But this is not enough: User 1 User 2 LOCK A [write A=1] UNLOCK A . . .. . .. . .. . . LOCK B [write B=2] UNLOCK B LOCK A [write A=3] UNLOCK A LOCK B [write B=4] UNLOCK B time What is wrong ?

41. 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

42. 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