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The Relational Model

The Relational Model

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The Relational Model

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  1. The Relational Model Class 2 Book Chapter 3 • Relational Data Model • Relational Query Language (DDL + DML) • Integrity Constraints (IC) • (From ER to Relational)

  2. Why Study the Relational Model? • Most widely used model in Commercial DBMSs: • Vendors: IBM, Informix, Microsoft, Oracle, Sybase… • “Legacy systems” in older models • E.G., IBM’s IMS • Competitor • Object-oriented model e.g. Informix Universal Server, UniSQL, O2, Oracle, DB2 • XML Model e.g. XML to Relational ( IBM, Oracle, SQL-Server, Rainbow, SilkRoute, XPERANTO) Native XML Engine ( Timber, Natix, etc.)

  3. Relational Database: Definitions • Relational database:a set of relations • Relation: made up of 2 parts: • Instance : a table, with rows (tuples) and columns (attributes). #Rows = cardinality, #attributes = degree / arity. • Schema :specifiesname of relation, plus name and type of each column. • E.G. • Students(sid: string, name: string, login: string,age: integer, gpa: real). • Think of a relation as a setof rows or tuples i.e., all rows are distinct (not required by commercial database)

  4. Example Instance of Students Relation • Cardinality = ?, degree = ? • Cardinality = 3, degree = 5. • All rows are distinct • Do all columns in a relation instance have to be distinct?

  5. Attribute Types / Domain • Each column of a relation has a name • Set of allowed values for each column is called domain of column • Domain specifies that values of the column must be drawn from the domain associated with the column – domain constraint • Column values are (normally) required to be atomic, i.e., indivisible • The special value null is a member of every domain • Null value causes complications in the definition of many operations • We shall ignore the effect of null values in our main presentation and consider their effect later

  6. Relations are Unordered • Order of tuples is irrelevant (tuples may be stored in an arbitrary order) • E.g., account relation with unordered tuples

  7. Database • A database consists of multiple relations • Information about an enterprise is broken up into parts, with each relation storing one part of the informationE.g.: account : stores information about accountsdepositor : stores information about which customer owns which account customer : stores information about customers • Storing all information as a single relation such as bank(account-number, balance, customer-name, ..)results in: • repetition of information (e.g., two customers own an account) • need for null values (e.g., represent a customer without an account) • Normalization theory (Chapter 7) deals with how to efficiently design relational schemas.

  8. Relational Query Languages (SQL) • Developed by IBM (system R) in the 1970s • Need for a standard since it is used by many vendors • Standards: • SQL-86 • SQL-89 (minor revision) • SQL-92 (major revision) • SQL-99 (major extensions, current standard)

  9. Relational Query Languages (SQL) • A major strength of the relational model: supports simple, powerful querying of data. • Queries can be written intuitively, and the DBMS is responsible for efficient evaluation. • The key: precise semantics for relational queries. • Allows the optimizer to extensively re-order operations, and still ensure that the answer does not change. • SQL = DDL + DML + …… (Chap 5)

  10. DDL ---- Creating Relations CREATE TABLE Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL) • Creates Students relation. Observe that type (domain) of each field is specified, and enforced by DBMS whenever tuples are added or modified. • As another example, Enrolled table holds information about courses that students take. CREATE TABLE Enrolled (sid: CHAR(20), cid: CHAR(20), grade: CHAR(2))

  11. DDL --- Destroying and Altering Relations DROP TABLE Students • Destroys the relation Students. The schema information and the tuples are deleted. ALTER TABLE Students ADD COLUMN firstYear: integer • The schema of Students is altered by adding a new field; every tuple in the current instance is extended with a null value in the new field.

  12. DML --- Query single relation • To find all 18 year old students, we can write: SELECT * FROM Students S WHERE S.age=18 • To find just names and logins, replace the first line: SELECT S.name, S.login

  13. DML --- Querying Multiple Relations SELECT S.name, E.cid FROM Students S, Enrolled E WHERE S.sid=E.sid AND E.grade=“A” • What does the following query compute? Given the following instances of Enrolled and Students: we get:

  14. DML --- Adding and Deleting Tuples • Can insert a single tuple using: INSERT INTO Students (sid, name, login, age, gpa) VALUES (53688, ‘Smith’, ‘smith@ee’, 18, 3.2) • Can delete all tuples satisfying some condition (e.g., name = Smith): DELETE FROM Students S WHERE S.name = ‘Smith’ • Powerful variants of these commands are available; more later!

  15. Integrity Constraints (ICs) • IC: condition that must be true for any instance of the database • ICs are specified when schema is defined. • ICs are checked when relations are modified. • A legalinstance of a relation is one that satisfies all specified ICs. • DBMS should not allow illegal instances.

  16. Integrity Constraints (ICs) • IC include: • Fundamental constraints : • Key • Foreign Key, • Domain Constraints • General constraints: • table constraints (single table) • assertions (several tables)

  17. Key Constraint • Two rules for Key constraints: • Two distinct tuples in a legal instance cannot have identical values in all columns of keys (unique) • No subset of the set of fields in a key is a unique identifier for a tuple (maximal) • Example: • “No two students can have the same student Id “ • “No two students can have the same student Id and name” CORE IDEA : Minimal subset of columns of the relation that uniquely identify the tuple.

  18. Keys • Let K  R • K is a superkeyof R if values for K are sufficient to identify a unique tuple of relation r(R) • Example: {customer-name, customer-street} and {customer-name} are both superkeys of relation Customer. NO two customers can possibly have the same name. • Set of all fields is a super key • K is a candidate key if K is minimal • Example: {customer-name} is a candidate key for Customer. - superkey - no subset of it is a superkey.

  19. Primary and Candidate Keys in SQL • Possibly many candidate keys(specified using UNIQUE), one of which is chosen as the primary key. CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid) ) CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade) )

  20. Primary and Candidate Keys in SQL CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid) ) “For a given student and course, there is a single grade.” vs. “Students can take only one course, and receive a single grade for that course; further, no two students in a course receive the same grade.” CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade) ) Used carelessly, an IC can prevent the storage of database instances that arise in practice!

  21. Foreign Keys, Referential Integrity • Foreign key : Set of fields in one relation that is used to "refer" to a tuple in another relation. • Like a `logical pointer’. • Foreign key : • FK in referencing relation must match PK of referenced relation. • Match = same number of columns, compatible data types (column names can be different). Enrolled (referencing relation) Students (referenced relation) Primary Key Foreign Key

  22. Foreign Keys in SQL • Only students listed in Students relation should be allowed to enroll for courses. CREATE TABLE Enrolled (sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students ) • If all foreign key constraints are enforced, referential integrity is achieved, i.e., no dangling references.

  23. Enforcing Referential Integrity • Consider Students and Enrolled; sid in Enrolled is a foreign key that references Students. • Insertion: What if a new Student tuple is inserted? • Insertion: What should be done if an Enrolled tuple with a non-existent student id is inserted? • Reject it Enrolled (referencing relation) Students (referenced relation) Primary Key Foreign Key

  24. Enforcing Referential Integrity Enrolled (referencing relation) Students (referenced relation) Deletion: What if an Enrolled tuple is deleted? Primary Key Foreign Key

  25. Enforcing Referential Integrity Enrolled (referencing relation) Students (referenced relation) Deletion: What if a Students tuple is deleted? • Cascading -- Also delete all Enrolled tuples that refer to it. • No Action -- Disallow deletion of a Students tuple that is referred to. • Set Default -- Set sid in Enrolled tuples that refer to it to a default sid. • Set Null -- Set sid in Enrolled tuples that refer to it to a special value null, denoting `unknown’ or `inapplicable’. (Not always applicable) • Similar if primary key of Students tuple is updated. Primary Key Foreign Key

  26. Referential Integrity in SQL • SQL/92 and SQL/99 support all 4 options on deletes and updates: • Default is NO ACTION (delete/update is rejected) • CASCADE (also delete all tuples that refer to deleted tuple) • SET NULL / SET DEFAULT (sets foreign key value of referencing tuple) CREATE TABLE Enrolled (sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students ON DELETE CASCADE ON UPDATE SET DEFAULT )

  27. Where do ICs Come From? • ICs are based upon semantics of real-world enterprise being described in database relations. • We can check a database instance to see if an IC is violated, • but we can NEVER infer that an IC is true by looking at an instance. • An IC is a statement about all possible instances! • From example, we know name is not a key, but the assertion that sid is a key is given to us. • Key and foreign key ICs are the most common; but more general ICs supported too in some systems.

  28. Views • A viewis just a relation, but we store a definition, rather than a set of tuples. CREATE VIEW YoungActiveStudents (name, sid, course) AS SELECT S.name, S.sid, E.cid FROM Students S, Enrolled E WHERE S.sid = E.sid • Views can be dropped using DROP VIEW command. • How to handle DROP TABLE if there’s a view on the table? • DROP TABLE { RESTRICT | CASCADE } 

  29. Views and Security • Views can be used to present necessary information (or a summary), while hiding details in underlying relation(s). • Given YoungStudents view only (not Students or Enrolled table) • User can find students who have enrolled, but not the gradesof the courses they got.

  30. Relational Model Terminology • Schema / Data Model • Relation /Relation Schema • Instance / Database • Schema Definition Language • Constraint Specification Language

  31. Relational Model: Summary • A tabular representation of data. • Simple and intuitive, currently the most widely used. • Integrity constraints can be specified by the DBA, based on application semantics. • DBMS checks for violations. • Two important ICs: primary and foreign keys • In addition, we always have domain constraints. • Powerful and ‘natural’ query languages exist.