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1、The Relational ModelClass 2Book Chapter 3 Relational Data Model Relational Query Language (DDL + DML) Integrity Constraints (IC) (From ER to Relational)Why Study the Relational Model? Most widely used model in Commercial DBMSs: Vendors: IBM, Informix, Microsoft, Oracle, Sybase“Legacy systems” in old

2、er models E.G., IBMs IMSCompetitorsObject-oriented model e.g., O2, but also Oracle, DB2XML Modele.g. Natix, but also IBM, OracleKey-Value Stores/NonSQLe.g. BigTable, MongoDB, etc. Relational Database: DefinitionsRelational database: a set of relationsRelation: made up of 2 parts:Instance : a table,

3、with rows (tuples) and columns (attributes). #Rows = cardinality, #attributes = degree / arity.Schema : specifies name 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 set of rows or tuplesi.e., al

4、l rows are distinct (not required by commercial database)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?Attribute Types / DomainEach column of a relation has a nameSet of a

5、llowed values for each column is called domain of columnDomain specifies that values of the column must be drawn from the domain associated with the column domain constraintColumn values are (normally) required to be atomic, i.e., indivisibleThe special value null is a member of every domain (ignore

6、 for now)Relations are Unordered Order of tuples is irrelevant (tuples may be stored in an arbitrary order) E.g., Account relation with unordered tuplesDatabaseA database consists of multiple relationsInformation about an enterprise is broken up into parts, with each relation storing one part of the

7、 informationE.g.: account : stores information about accounts depositor : stores information about which customer owns which account customer : stores information about customersStoring all information as a single relation such as bank(account-number, balance, customer-name, .)results in:repetition

8、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.Relational Query Languages (SQL)Developed by IBM (system R) in the 1970sNeed for a standard

9、since it is used by many vendorsStandards: SQL-86SQL-89 (minor revision)SQL-92 (major revision)SQL-99 (major extensions, current standard). . .Relational Query Languages (SQL)A major strength of the relational model: supports simple, powerful querying of data. Queries can be written intuitively, and

10、 the DBMS is responsible for efficient evaluation.The key: precise semantics for relational queries.Enables: optimizer re-orders operations, yet ensures that the answer does not change.SQL = DDL + DML + (Chap 5)DDL - Creating RelationsCreates Students relation. Observe that type (domain) of each fie

11、ld 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 Students(sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL) CREATE TABLE Enrolled(sid: CHAR(20), cid:

12、CHAR(20), grade: CHAR(2) DDL - Destroying and Altering RelationsDestroys the relation Students. The schema information and the tuples are deleted.DROP TABLE Students 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

13、field.ALTER TABLE Students ADD COLUMN firstYear: integerDML - Query single relationTo find all 18 year old students, we can write:SELECT *FROM Students SWHERE S.age=18To find just names and logins, replace the first line:SELECT S.name, S.loginFROM Students SWHERE S.age=18 DML - Querying Multiple Rel

14、ationsWhat does the following query compute?SELECT S.name, E.cidFROM Students S, Enrolled EWHERE S.sid=E.sid AND E.grade=“A”Given the following instances of Enrolled and Students:we get:DML - Adding and Deleting TuplesCan insert a single tuple using:INSERT INTO Students (sid, name, login, age, gpa)V

15、ALUES (53688, Smith, smithee, 18, 3.2)Can delete all tuples satisfying some condition (e.g., name = Smith):DELETE FROM Students SWHERE S.name = Smith Powerful variants of these commands are available; more later!Integrity Constraints (ICs)IC: condition that must be true for any instance of the datab

16、aseICs are specified when schema is defined.ICs are checked when relations are modified.A legal instance of a relation is one that satisfies all specified ICs. DBMS should not allow illegal instances.Integrity Constraints (ICs)IC include: Fundamental constraints : Key Foreign Key, Domain Constraints

17、General constraints: table constraints (single table)assertions (several tables)Key ConstraintTwo 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 (

18、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. KeysLet K RK is a superkey of R if values for K are sufficient to identify a unique tuple of r

19、elation 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 keyK is a candidate key if K is minimalExample: customer-name is a candidate key for Customer.- if superkey, a

20、nd - no subset of it is a superkey.Primary and Candidate Keys in SQLPossibly 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

21、), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade) )Primary and Candidate Keys in SQLCREATE 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 g

22、rade 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!Foreign Keys, Refere

23、ntial IntegrityForeign 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 differen

24、t).Enrolled (referencing relation)Students (referenced relation)Foreign KeyPrimary KeyForeign Keys in SQLOnly 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) REFERE

25、NCES Students ) If all foreign key constraints are enforced, referential integrity is achieved, i.e., no dangling references.Enforcing Referential Integritysid in Enrolled is a foreign key that references Students.Insertion: What if a new Student tuple is inserted? Insertion: What should be done if

26、an Enrolled tuple with a non-existent student id is inserted? Reject itEnrolled (referencing relation)Students (referenced relation)Foreign KeyPrimary KeyEnforcing Referential IntegrityDeletion: What if an Enrolled tuple is deleted?Enrolled (referencing relation)Students (referenced relation)Foreign

27、 KeyPrimary KeyEnforcing Referential IntegrityDeletion: 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.Se

28、t 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.Enrolled (referencing relation)Students (referenced relation)Foreign KeyPrimary KeyEnforcing Referential Integrit

29、yConsider Students and Enrolled; sid in Enrolled is a foreign key that references Students.Insertion: What should be done if an Enrolled tuple with a non-existent student id is inserted? (Reject it!)Deletion: What should be done if a Students tuple is deleted?Cascading - Also delete all Enrolled tup

30、les 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 applicab

31、le)Similar if primary key of Students tuple is updated.Referential Integrity in SQLSQL/99 supports 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 referencin

32、g tuple)CREATE TABLE Enrolled (sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students (sid)ON DELETE CASCADEON UPDATE SET DEFAULT )Where do ICs Come From?ICs are based upon semantics of real-world enterprise being described in database relations. Can

33、we check a database instance to see if an IC is violated ?Can we infer that an IC is true by looking at a database 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 by domain knowledge !Key and foreign key ICs are the most common; but more general ICs supported in some systems.Views Usef