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The Relational Model Class 2 Book Chapter 3 • • • • Relational Data Model Relational Query Language (DDL + DML) Integrity Constraints (IC) (From ER to Relational) Relational Data Model Relational Query Language Integrity Constraints ER to Relational Why Study the Relational Model? Most widely used model in Commercial DBMSs: “Legacy systems” in older models Vendors: IBM, Informix, Microsoft, Oracle, Sybase… E.G., IBM’s IMS Competitors Object-oriented model e.g., O2, but also Oracle, DB2 XML Model e.g. Natix, but also IBM, Oracle Key-Value Stores/NonSQL e.g. BigTable, MongoDB, etc. 2 Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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 : 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 tuples i.e., all rows are distinct (not required by commercial database) 3 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Example Instance of Students Relation sid 53666 53688 53650 name login Jones jones@cs Smith smith@eecs Smith smith@math age 18 18 19 gpa 3.4 3.2 3.8 Cardinality = ?, degree = ? Cardinality = 3, degree = 5. All rows are distinct Do all columns in a relation instance have to be distinct? 4 Attribute Types / Domain Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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 (ignore for now) 5 Relations are Unordered Relational Data Model Relational Query Language Integrity Constraints ER to Relational Order of tuples is irrelevant (tuples may be stored in an arbitrary order) E.g., Account relation with unordered tuples 6 Database Relational Data Model Relational Query Language Integrity Constraints ER to Relational A database consists of multiple relations Information about an enterprise is broken up into parts, with each relation storing one part of the information E.g.: account : stores information about accounts depositor : 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. 7 Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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) ... 8 Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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. Enables: optimizer re-orders operations, yet ensures that the answer does not change. SQL = DDL + DML + …… (Chap 5) 9 Relational Data Model Relational Query Language Integrity Constraints ER to Relational DDL ---- Creating Relations 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 Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL) CREATE TABLE Enrolled (sid: CHAR(20), cid: CHAR(20), grade: CHAR(2)) 10 Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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. 11 DML --- Query single relation Relational Data Model Relational Query Language Integrity Constraints ER to Relational To find all 18 year old students, we can write: SELECT * FROM Students S WHERE S.age=18 sid name 53666 Jones login jones@cs age gpa 18 3.4 53688 Smith smith@ee 18 3.2 •To find just names and logins, replace the first line: SELECT S.name, S.login FROM Students S WHERE S.age=18 12 Relational Data Model Relational Query Language Integrity Constraints ER to Relational DML --- Querying Multiple Relations What does the following query compute? SELECT S.name, E.cid FROM Students S, Enrolled E WHERE S.sid=E.sid AND E.grade=“A” Given the following instances of Enrolled and Students: sid 53666 53688 53650 name login age gpa Jones jones@cs 18 3.4 Smith smith@eecs 18 3.2 Smith smith@math 19 3.8 sid 53831 53831 53650 53666 cid grade Carnatic101 C Reggae203 B Topology112 A History105 B we get: S.name E.cid Smith Topology112 13 Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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! 14 Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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 legal instance of a relation is one that satisfies all specified ICs. • DBMS should not allow illegal instances. 15 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Integrity Constraints (ICs) IC include: • Fundamental constraints : • Key • Foreign Key, • Domain Constraints • General constraints: • table constraints (single table) • assertions (several tables) 16 Key Constraint Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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. 17 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Keys Let K R K is a superkey of 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. - if superkey, and - no subset of it is a superkey. 18 Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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) ) 19 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Primary and Candidate Keys in SQL “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,cid) ) 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! 20 Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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) sid 53666 53666 53650 53666 cid grade Carnatic101 C Reggae203 B Topology112 A History105 B Foreign Key Students (referenced relation) sid 53666 53688 53650 name login Jones jones@cs Smith smith@eecs Smith smith@math age 18 18 19 gpa 3.4 3.2 3.8 Primary Key 21 Relational Data Model Relational Query Language Integrity Constraints ER to Relational 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. 22 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Enforcing Referential Integrity sid in Enrolled is a foreign key that references Students. Enrolled (referencing relation) sid 53666 53666 53650 53666 cid grade Carnatic101 C Reggae203 B Topology112 A History105 B Foreign Key Students (referenced relation) sid 53666 53688 53650 name login Jones jones@cs Smith smith@eecs Smith smith@math age 18 18 19 gpa 3.4 3.2 3.8 Primary Key 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 23 Enforcing Referential Integrity Enrolled (referencing relation) sid 53666 53666 53650 53666 cid grade Carnatic101 C Reggae203 B Topology112 A History105 B Foreign Key Relational Data Model Relational Query Language Integrity Constraints ER to Relational Students (referenced relation) sid 53666 53688 53650 name login Jones jones@cs Smith smith@eecs Smith smith@math age 18 18 19 gpa 3.4 3.2 3.8 Primary Key Deletion: What if an Enrolled tuple is deleted? 24 Enforcing Referential Integrity Enrolled (referencing relation) sid 53666 53666 53650 53666 cid grade Carnatic101 C Reggae203 B Topology112 A History105 B Foreign Key Relational Data Model Relational Query Language Integrity Constraints ER to Relational Students (referenced relation) sid 53666 53688 53650 name login Jones jones@cs Smith smith@eecs Smith smith@math age 18 18 19 gpa 3.4 3.2 3.8 Primary Key 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. 25 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Referential Integrity in SQL SQL/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 referencing 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 CASCADE ON UPDATE SET DEFAULT ) 27 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Where do ICs Come From? ICs are based upon semantics of real-world enterprise being described in database relations. Can 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. 28 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Views – Useful For Design A view is 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 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Views and Security Views can be used to present necessary information (or provide a summary), while hiding details in underlying relation(s). For example: Given YoungStudents view only (not Students or Enrolled table), the user can find students who have enrolled, but not the grades of the courses they got. 30 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Relational Model Terminology Schema / Data Model Relation /Relation Schema Instance / Database Schema Definition Language 31 Relational Data Model Relational Query Language Integrity Constraints ER to Relational Relational Model: Summary Tabular representation of data. Simple and intuitive, yet 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, ‘natural’ query language exist. 32