Transactions, Relational Algebra, XML February 11 th, 2004

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 !

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

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

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

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

Isolation Levels in SQL 1.“Dirty reads” SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED 2.“Committed reads” SET TRANSACTION ISOLATION LEVEL READ COMMITTED 3.“Repeatable reads” SET TRANSACTION ISOLATION LEVEL REPEATABLE READ 4.Serializable transactions (default): SET TRANSACTION ISOLATION LEVEL SERIALIZABLE Reading assignment: chapter 8.6

Relational Algebra Formalism for creating new relations from existing ones Its place in the big picture: Declartive query language Algebra Implementation SQL, relational calculus Relational algebra Relational bag algebra

Relational Algebra Five operators: –Union:  –Difference: - –Selection:  –Projection:  –Cartesian Product:  Derived or auxiliary operators: –Intersection, complement –Joins (natural,equi-join, theta join, semi-join) –Renaming: 

1. Union and 2. Difference R1  R2 Example: –ActiveEmployees  RetiredEmployees R1 – R2 Example: –AllEmployees -- RetiredEmployees

What about Intersection ? It is a derived operator R1  R2 = R1 – (R1 – R2) Also expressed as a join (will see later) Example –UnionizedEmployees  RetiredEmployees

3. Selection Returns all tuples which satisfy a condition Notation:  c (R) Examples –  Salary > (Employee) –  name = “Smithh” (Employee) The condition c can be =,, , <>

Find all employees with salary more than $40,000.  Salary > (Employee)

4. Projection Eliminates columns, then removes duplicates Notation:  A1,…,An (R) Example: project social-security number and names: –  SSN, Name (Employee) –Output schema: Answer(SSN, Name)

 SSN, Name (Employee)

5. Cartesian Product Each tuple in R1 with each tuple in R2 Notation: R1  R2 Example: –Employee  Dependents Very rare in practice; mainly used to express joins

Relational Algebra Five operators: –Union:  –Difference: - –Selection:  –Projection:  –Cartesian Product:  Derived or auxiliary operators: –Intersection, complement –Joins (natural,equi-join, theta join, semi-join) –Renaming: 

Renaming Changes the schema, not the instance Notation:  B1,…,Bn (R) Example: –  LastName, SocSocNo (Employee) –Output schema: Answer(LastName, SocSocNo)

Renaming Example Employee NameSSN John Tony LastNameSocSocNo John Tony  LastName, SocSocNo (Employee)

Natural Join Notation: R1 ⋈ R2 Meaning: R1 ⋈ R2 =  A (  C (R1  R2)) Where: –The selection  C checks equality of all common attributes –The projection eliminates the duplicate common attributes

Natural Join Example Employee NameSSN John Tony Dependents SSNDname Emily Joe NameSSNDname John Emily Tony Joe Employee Dependents =  Name, SSN, Dname (  SSN=SSN2 (Employee x  SSN2, Dname (Dependents))

Natural Join R= S= R ⋈ S= AB XY XZ YZ ZV BC ZU VW ZV ABC XZU XZV YZU YZV ZVW

Natural Join Given the schemas R(A, B, C, D), S(A, C, E), what is the schema of R ⋈ S ? Given R(A, B, C), S(D, E), what is R ⋈ S ? Given R(A, B), S(A, B), what is R ⋈ S ?

Theta Join A join that involves a predicate R1 ⋈  R2 =   (R1  R2) Here  can be any condition

Eq-join A theta join where  is an equality R1 ⋈ A=B R2 =  A=B (R1  R2) Example: –Employee ⋈ SSN=SSN Dependents Most useful join in practice

Semijoin R ⋉ S =  A1,…,An (R ⋈ S) Where A 1, …, A n are the attributes in R Example: –Employee ⋉ Dependents

Semijoins in Distributed Databases Semijoins are used in distributed databases SSNName... SSNDnameAge... Employee Dependents network Employee ⋈ ssn=ssn (  age>71 (Dependents)) T =  SSN  age>71 (Dependents) R = Employee ⋉ T Answer = R ⋈ Dependents

Complex RA Expressions Person Purchase Person Product  name=fred  name=gizmo  pid  ssn seller-ssn=ssnpid=pidbuyer-ssn=ssn  name

Operations on Bags A bag = a set with repeated elements All operations need to be defined carefully on bags {a,b,b,c}  {a,b,b,b,e,f,f}={a,a,b,b,b,b,b,c,e,f,f} {a,b,b,b,c,c} – {b,b,c,c,c,d} = {a,b}  C (R): preserve the number of occurrences  A (R): no duplicate elimination Cartesian product, join: no duplicate elimination Important ! Relational Engines work on bags, not sets ! Reading assignment: 5.3 – 5.4

Finally: RA has Limitations ! Cannot compute “transitive closure” Find all direct and indirect relatives of Fred Cannot express in RA !!! Need to write C program Name1Name2Relationship FredMaryFather MaryJoeCousin MaryBillSpouse NancyLouSister

XMLXML

XML eXtensible Markup Language XML 1.0 – a recommendation from W3C, 1998 Roots: SGML (a very nasty language). After the roots: a format for sharing data

Why XML is of Interest to Us XML is just syntax for data –Note: we have no syntax for relational data –But XML is not relational: semistructured This is exciting because: –Can translate any data to XML –Can ship XML over the Web (HTTP) –Can input XML into any application –Thus: data sharing and exchange on the Web

XML Data Sharing and Exchange application relational data Transform Integrate Warehouse XML DataWEB (HTTP) application legacy data object-relational Specific data management tasks

From HTML to XML HTML describes the presentation

HTML Bibliography Foundations of Databases Abiteboul, Hull, Vianu Addison Wesley, 1995 Data on the Web Abiteoul, Buneman, Suciu Morgan Kaufmann, 1999 Bibliography Foundations of Databases Abiteboul, Hull, Vianu Addison Wesley, 1995 Data on the Web Abiteoul, Buneman, Suciu Morgan Kaufmann, 1999

XML Foundations… Abiteboul Hull Vianu Addison Wesley 1995 … Foundations… Abiteboul Hull Vianu Addison Wesley 1995 … XML describes the content