The Relational Model Chapter 3

Transcription

1 The Relational Model Chapter 3 How is data represented in the relational model? What integrity constraints can be expressed? How can be data created and modified? How can data be manipulated and queried? How can we create, modify and query tables using SQL? How do we obtain a relational db design from an ER diagram? What are views and where are they used? Unit 3/Fall03/Melikyan 1

2 Why Study the Relational Model? Most widely used model. Vendors: IBM, Informix, Microsoft, Oracle, Sybase, etc. Legacy systems in older models E.G., IBM s IMS Recent competitor: object-oriented model ObjectStore, Versant, Ontos A synthesis emerging: object-relational model u Informix Universal Server, UniSQL, O2, Oracle, DB2 Unit 3/Fall03/Melikyan 2

3 Data Definition Language (DDL) The standard language for creating, manipulating, and querying data in relational DBMS. We will discuss the concept of relation and show how to create relation using the SQL language We will use DDL to specify conditions that must be satisfied by the data (ICs) - integrity constraints Then we turn to the mechanism for accessing data from DB and introduce the querying features of SQL Unit 3/Fall03/Melikyan 3

4 Relational Database: Definitions Relational database: a set of relations Relation: made up of 2 parts:instance : a table, with rows and columns. # Rows = cardinality, # Fields = degree / arity. Schema :the relation schema specifies the relations name, the name of each field and the domain of each field: Students(sid: string, name: string, login: string, age: integer, gpa: real). Can think of a relation as a set of rows or tuples (i.e., all rows are distinct). Unit 3/Fall03/Melikyan 4

5 Example Instance of Students Relation An instance of relation is a set of tuples, also called records. An instence of the students relation sid name login age gpa Jones jones@cs Smith smith@eecs Smith smith@math Cardinality = 3, degree = 5, all rows distinct Do all columns in a relation instance have to be distinct? Unit 3/Fall03/Melikyan 5

6 Creating Relations in SQL Creates the Students relation. Observe that the type (domain) of each field is specified, and enforced by the DBMS whenever tuples are added or modified. 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)) As another example, the Enrolled table holds information about courses that students take. Unit 3/Fall03/Melikyan 6

7 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! Unit 3/Fall03/Melikyan 7

8 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. Unit 3/Fall03/Melikyan 8

9 Update We can modify the column values in existing row using the update command UPDATE Stidents S SET S.age = S.age + 1, S.gpa = S.gpa 1 WHERE S.sid = Where clause is applied first to determine which roe are to be modified. Unit 3/Fall03/Melikyan 9

10 Relational Query Languages 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. Unit 3/Fall03/Melikyan 10

11 The SQL Query Language 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, current standard) SQL-99 (major extensions) Unit 3/Fall03/Melikyan 11

12 The SQL Query Language To find all 18 year old students, we can write: SELECT * FROM Students S WHERE S.age=18 sid name login age gpa Jones jones@cs Smith smith@ee To find just names and logins, replace the first line: SELECT S.name, S.login Unit 3/Fall03/Melikyan 12

13 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 instance of Enrolled (is this possible if the DBMS ensures referential integrity?): Unit 3/Fall03/Melikyan 13

14 Who Got an A? sid name login age gpa Jones Smith Smith Result sid cid grade C O M P3300 C M A TH 1100 B Topology112 A H istory105 B S.name Smith E.cid Topology112 Unit 3/Fall03/Melikyan 14

15 Integrity Constraints (ICs) IC: condition that must be true for any instance of the database; e.g., domain constraints. 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. If the DBMS checks ICs, stored data is more faithful to realworld meaning. Avoids data entry errors, too! Unit 3/Fall03/Melikyan 15

16 Primary Key Constraints A set of fields is a key for a relation if 1. No two distinct tuples can have same values in all key fields, and 2. This is not true for any subset of the key. Part 2 false? A superkey. If there s >1 key for a relation, one of the keys is chosen (by DBA) to be the primary key. E.g., sid is a key for Students. (What about name?) The set {sid, gpa} is a superkey. Unit 3/Fall03/Melikyan 16

17 Primary and Candidate Keys in SQL Possibly many candidate keys (specified using UNIQUE), one of which is chosen as the primary key. CREATE TABLE Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL, UNIQUE (name, age), CONSTRAINT StudentsKey PRIMARY KEY (sid)) Unit 3/Fall03/Melikyan 17

18 More About Keys 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. Used carelessly, an IC can prevent the storage of Database instances that arise in practice! 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) ) Unit 3/Fall03/Melikyan 18

19 Foreign Keys, Referential Integrity Foreign key : Set of fields in one relation that is used to `refer to a tuple in another relation. (Must correspond to primary key of the second relation.) Like a `logical pointer. E.g. sid is a foreign key referring to Students: Students(sid: string, name: string, login: string, age: integer, gpa: real). Enrolled(sid: string, cid: string, grade: string) v v If all foreign key constraints are enforced, referential integrity is achieved, i.e., no dangling references. Can you name a data model w/o referential integrity? Links in HTML! Unit 3/Fall03/Melikyan 19

20 Foreign Keys in SQL Only students listed in the 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 ) Enrolled sid cid grade Carnatic101 C Reggae203 B Topology112 A History105 B Students sid name login age gpa Jones jones@cs Smith smith@eecs Smith smith@math Unit 3/Fall03/Melikyan 20

21 Enforcing Referential Integrity Consider Students and Enrolled; sid in Enrolled is a foreign key that references Students. What should be done if an Enrolled tuple with a non-existent student id is inserted? (Reject it!) What should be done if a Students tuple is deleted? Also delete all Enrolled tuples that refer to it. Disallow deletion of a Students tuple that is referred to. Set sid in Enrolled tuples that refer to it to a default sid. (In SQL, also: Set sid in Enrolled tuples that refer to it to a special value null, denoting `unknown or `inapplicable.) Similar if primary key of Students tuple is updated. Unit 3/Fall03/Melikyan 21

22 Referential Integrity in SQL/92 SQL/92 supports 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) 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 ) SET NULL / SET DEFAULT (sets foreign key value of referencing tuple) Unit 3/Fall03/Melikyan 22

23 Where do ICs Come From? ICs are based upon the semantics of the real-world enterprise that is being described in the 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; more general ICs supported too. Unit 3/Fall03/Melikyan 23

24 Transactions and Constraints By default, a constraint is checked at the end of SQL statement CREATE TABLE Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, honors CHAR(10) NOT NULL, gpa: REAL, PRIMARY KEY (sid), FOREIGN KEY (honors) References Courses (cid) ) CREATE TABLE Courses (cid: CHAR(10), cname: CHAR(10), credits: INTEGER, grader: CHAR(20) NOT NULL, PRIMARY KEY (cid), FOREIGN KEY (grader) References Students (sid) ) SQL allows a constraint to be in DEFERRED and IMMEDIATE mode SET CONSTRAINT Foo DEFERRED. It is checked at commit time Unit 3/Fall03/Melikyan 24

25 Views A view is just a relation, but we store a definition, rather than a set of tuples. CREATE VIEW YoungActiveStudents (name, grade) AS SELECT S.name, E.grade FROM Students S, Enrolled E WHERE S.sid = E.sid and S.age<21 Views can be dropped using the DROP VIEW command. How to handle DROP TABLE if there s a view on the table? DROP TABLE command has options to let the user specify this. Unit 3/Fall03/Melikyan 25

26 Views and Security Views can be used to present necessary information (or a summary), while hiding details in underlying relation(s). Given YoungStudents, but not Students or Enrolled, we can find students s who have are enrolled, but not the cid s of the courses they are enrolled in. Unit 3/Fall03/Melikyan 26

27 Logical DB Design: ER to Relational Entity sets to tables. ssn name lot Employees CREATE TABLE Employees (ssn CHAR(11), name CHAR(20), lot INTEGER, PRIMARY KEY (ssn)) Unit 3/Fall03/Melikyan 27

28 Relationship Sets to Tables (Without constraints) In translating a relationship set to a relation, attributes of the relation must include: v Keys for each participating entity set (as foreign keys). v This set of attributes forms a superkey for the relation. v All descriptive attributes. CREATE TABLE Works_In( ssn CHAR(1), did INTEGER, since DATE, PRIMARY KEY (ssn, did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments) Unit 3/Fall03/Melikyan 28

29 Review: Key Constraints Each dept has at most one manager, according to the key constraint on Manages. ssn name lot since did dname budget Employees Manages Departments Translation to relational model? 1-to-1 1-to Many Many-to-1 Many-to-Many Unit 3/Fall03/Melikyan 29

30 Translating ER Diagrams with Key Constraints Map relationship to a table: Note that did is the key now! Separate tables for Employees and Departments. Since each department has a unique manager, we could instead combine Manages and Departments. CREATE TABLE Manages( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments) CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees) Unit 3/Fall03/Melikyan 30

31 Review: Participation Constraints Does every department have a manager? v v If so, this is a participation constraint: the participation of Departments in Manages is said to be total (vs. partial). Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!) name since dname ssn lot did budget Employees Manages Departments Works_In since Unit 3/Fall03/Melikyan 31

32 Participation Constraints in SQL We can capture participation constraints involving one entity set in a binary relationship, but little else (without resorting to CHECK constraints). CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE NO ACTION) Unit 3/Fall03/Melikyan 32

33 Review: Weak Entities A weak entity can be identified uniquely only by considering the primary key of another (owner) entity. v v Owner entity set and weak entity set must participate in a one-tomany relationship set (1 owner, many weak entities). Weak entity set must have total participation in this identifying relationship set. name ssn lot cost pname age Employees Policy Dependents Unit 3/Fall03/Melikyan 33

34 Translating Weak Entity Sets Weak entity set and identifying relationship set are translated into a single table. When the owner entity is deleted, all owned weak entities must also be deleted. CREATE TABLE Dep_Policy ( pname CHAR(20), age INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (pname, ssn), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE) Unit 3/Fall03/Melikyan 34

35 Review: ISA Hierarchies ssn name lot Employees hourly_wages hours_worked vas in C++, or other PLs, attributes are inherited. ISA contractid vif we declare A ISA B, every A entity is also considered to be a B entity. Hourly_Emps Contract_Emps Overlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity? (Allowed/disallowed) Covering constraints: Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no) Unit 3/Fall03/Melikyan 35

36 Translating ISA Hierarchies to Relations General approach: 3 relations: Employees, Hourly_Emps and Contract_Emps. u Hourly_Emps: Every employee is recorded in Employees. For hourly emps, extra info recorded in Hourly_Emps (hourly_wages, hours_worked, ssn); must delete Hourly_Emps tuple if referenced Employees tuple is deleted). u Queries involving all employees easy, those involving just Hourly_Emps require a join to get some attributes. Alternative: Just Hourly_Emps and Contract_Emps. Hourly_Emps: ssn, name, lot, hourly_wages, hours_worked. Each employee must be in one of these two subclasses. Unit 3/Fall03/Melikyan 36

37 Review: Binary vs. Ternary Relationships If each policy is owned by just 1 ssn name lot pname age employee: Employees Covers Dependents Key constraint on Policies would mean policy can only cover 1 dependent! What are the additional constraints in the 2nd diagram? Bad design name ssn lot Employees Policies policyid cost pname age Dependents Purchaser Better design policyid Beneficiary Policies Unit 3/Fall03/Melikyan 37 cost

38 Binary vs. Ternary Relationships (Contd.) CREATE TABLE Policies ( The key constraints policyid INTEGER, allow us to combine cost REAL, Purchaser with ssn CHAR(11) NOT NULL, Policies and PRIMARY KEY (policyid). Beneficiary with FOREIGN KEY (ssn) REFERENCES Employees, Dependents. ON DELETE CASCADE) Participation constraints lead to NOT NULL constraints. What if Policies is a weak entity set? CREATE TABLE Dependents ( pname CHAR(20), age INTEGER, policyid INTEGER, PRIMARY KEY (pname, policyid). FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE) Unit 3/Fall03/Melikyan 38

39 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. Rules to translate ER to relational model Unit 3/Fall03/Melikyan 39

40 Homework#2 Problem 2.2 A university database contains information about Professors (identifed by social security number, or SSN) and Courses (identifed by courseid). Professors teach courses; each of the following situations concerns the Teaches relationship set. For each situation, draw an ER diagram that describes it (assuming that no further constraints hold). Unit 3/Fall03/Melikyan 40

41 Solutions for HW2 1. Professors can teach the same course in several semesters, and each offering must be recorded. Unit 3/Fall03/Melikyan 41

42 2. Professors can teach the same course in several semesters, and only the most recent such offering needs to be recorded. (Assume this condition applies in all subsequent questions.) Unit 3/Fall03/Melikyan 42

43 3. Every professor must teach some course. Unit 3/Fall03/Melikyan 43

44 4. Every professor teaches exactly one course (no more, no less). Unit 3/Fall03/Melikyan 44

45 5. Every professor teaches exactly one course (no more, no less), and every course must be taught by some professor. Unit 3/Fall03/Melikyan 45

46 Now suppose that certain courses can be taught by a team of professors jointly, but it is possible that no one professor in a team can teach the course. Model this situation, introducing additional entity sets and relationship sets if necessary. Unit 3/Fall03/Melikyan 46

47 Solutions for 2.3 Unit 3/Fall03/Melikyan 47

48 Solution for 2.4 Unit 3/Fall03/Melikyan 48

49 HW #3 HOMEWORK: Answer the following questions from your textbook, page Ex 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.8,3.12 Assigned 09/ 25 /03 Due 10/02/03 SUBMIT: hard copy by the beginning of class Unit 3/Fall03/Melikyan 49