1. Database Management Systems 1 Raghu Ramakrishnan The Relational Model Chapter 3 Instructor: Xin Zhang

2. Database Management Systems 2 Raghu Ramakrishnan Why Study the Relational Model? v Most widely used model. –Vendors: IBM, Informix, Microsoft, Oracle, Sybase, etc. v “Legacy systems” in older models –e.g., IBM’s IMS v Recent competitor: Object-Oriented model –ObjectStore, Versant, Ontos –a synthesis emerging: object-relational model u Informix Universal Server, UniSQL, O2, Oracle, DB2

3. Database Management Systems 3 Raghu Ramakrishnan Relational Database: Definitions v Relational database: a set of relations. v Relation: consist of 2 parts: – Instance : a table, with rows and columns. #rows = cardinality, #fields = degree – Schema : specifies name of relation, plus name, type of each column, and their domain. u E.g. Students( sid : string, name : string, login : string, age : integer, gpa : real) v Can think of a relation as a set of rows or tuples. (i.e., all rows are distinct )

4. Database Management Systems 4 Raghu Ramakrishnan Example Instance of Students Relation v Cardinality = 3, degree = 5, all rows distinct v Do all columns in a relation instance have to be distinct?

5. Database Management Systems 5 Raghu Ramakrishnan Example Instance of Bank Account v Cardinality = 6, degree = 4, all rows distinct AcctName Balance 1005Jones $1023.22 1002Smith $22.43 1003Smith $11000.12 1008Green$1077.23 Address 11 First St 12 First St 14 Fourth St 1010Gates $5,000,000,000.00 20 Tenth St 1012Smith $443.77 15 Second St

6. Database Management Systems 6 Raghu Ramakrishnan Advantages of Relational Database v A major strength of the relational model: supports simple, powerful querying of data. v Queries can be written intuitively, and the DBMS is responsible for efficient evaluation. v Query Language (SQL) is well-defined.

7. Database Management Systems 7 Raghu Ramakrishnan Integrity Constraints (ICs) v 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. v A legal instance of a relation is one that satisfies all specified ICs. –DBMS should not allow illegal instances. v If the DBMS checks ICs, stored data is more faithful to real-world meaning. –Avoids data entry errors, too! Domain, primary key, foreign key constraints

8. Database Management Systems 8 Raghu Ramakrishnan Where do ICs Come From? v ICs are based upon the semantics of the real- world enterprise that is being described in the database relations. v 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! v Key and foreign key ICs are the most common; more general ICs supported too.

9. Database Management Systems 9 Raghu Ramakrishnan Primary Key Constraints v 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. v E.g., sid is a key for Students. (What about name ?) The set { sid, gpa } is a superkey.

10. Database Management Systems 10 Raghu Ramakrishnan Primary and Candidate Keys in SQL v 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) ) v “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) )

11. Database Management Systems 11 Raghu Ramakrishnan Primary and Candidate Keys in SQL (continued) v For Students relation with SID as the primary key CREATE TABLE Students (sid CHAR(20), name CHAR(20), login CHAR (10), age INTEGER, gpa REAL, PRIMARY KEY (sid) ) v Are there any separate fields or combinations of fields which also are candidates for primary key? –How about login? –How about age? –How about age & gpa?

12. Database Management Systems 12 Raghu Ramakrishnan Foreign Keys, Referential Integrity v 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 ’. v E.g. sid is a foreign key referring to Students: –Enrolled( sid : string, cid : string, grade : string) –If all foreign key constraints are enforced, referential integrity is achieved, i.e., no dangling references.

13. Database Management Systems 13 Raghu Ramakrishnan Foreign Keys in SQL v 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 (cid) REFERENCES Courses FOREIGN KEY (sid) REFERENCES Students ) Enrolled Students

14. Database Management Systems 14 Raghu Ramakrishnan Foreign Keys in SQL (continued) v Bank Account Example - Only allow accounts opened for customers listed in the customer information relation v First, an instance of customer information Name Phone Jones 6032223456 Smith 6033335567 Address 11 First St 12 First St Green781444789014 Fourth St Gates 1223336789 11 First St Smith 6039788765 12 Second St Email Jones@cs Smith@cs Green@cs Gates@ms Smith@ee

15. Database Management Systems 15 Raghu Ramakrishnan Foreign Keys in SQL (continued) v Creates the customer information relation CREATE TABLE Customer_Info (name CHAR (20), addr CHAR(40), phone CHAR (10), email char (40), PRIMARY KEY (name, addr)) v Now create the bank account relation with a foreign key CREATE TABLE Bank_Acct (acct CHAR (4), name CHAR (20), address char (40), balance REAL, PRIMARY KEY (acct), Foreign Key (name, address) references Customer_Info)

16. Database Management Systems 16 Raghu Ramakrishnan Foreign Keys in SQL (continued) Name Phone Jones6032223456 Smith 6033335567 Address 11 First St 12 First St Green781444789014 Fourth St Gates 1223336789 11 First St Smith 6039788765 12 Second St Email Jones@cs Smith@cs Green@cs Gates@ms Smith@ee AcctName Balance 1005Jones$1023.22 1002Smith $22.43 1003Smith $11000.12 1008Green$1077.23 Address 11 First St 12 First St 14 Fourth St 1010Gates $5,000,000,000.00 20 Tenth St 1012Smith $443.77 15 Second St

17. Database Management Systems 17 Raghu Ramakrishnan Enforcing Referential Integrity v Consider Students and Enrolled; sid in Enrolled is a foreign key that references Students. v What should be done if an Enrolled tuple with a non- existent student id is inserted? ( Reject it! ) v 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’.) v Similar if primary key of Students tuple is updated.

18. Database Management Systems 18 Raghu Ramakrishnan Referential Integrity in SQL/92 v 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) – 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 )

19. Database Management Systems 19 Raghu Ramakrishnan The SQL Query Language v Developed by IBM (system R) in the 1970s v Need for a standard since it is used by many vendors v Standards: – SQL-86 – SQL-89 (minor revision) – SQL-92 (major revision, current standard) – SQL-99 (major extensions)

20. Database Management Systems 20 Raghu Ramakrishnan The SQL Query Language a.How to generate (create) a relation (table) in database? b. How to modify the table (relation) in database? delete and insert? or modify one attribute? c. How to realize the access (query) relation in database? How to support database operations by using SQL?

21. Database Management Systems 21 Raghu Ramakrishnan Creating Relations in SQL v 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 )

22. Database Management Systems 22 Raghu Ramakrishnan CREATE TABLE Enrolled (sid CHAR(20), cid CHAR(20), grade CHAR (2)) v As another example, the Enrolled table holds information about courses that students take.

23. Database Management Systems 23 Raghu Ramakrishnan Creating Relations in SQL (cont) v Creates the Bank Account relation. CREATE TABLE Bank_Acct (acct CHAR(4), name CHAR(20), address char(40), balance REAL ) AcctName Balance 1005Jones $1023.22 1002Smith $22.43 1003Smith $11000.12 1008Green$1077.23 Address 11 First St 12 First St 14 Fourth St 1010Gates $5,000,000,000.00 20 Tenth St 1025Blue $2344.22 14 Third St

24. Database Management Systems 24 Raghu Ramakrishnan Destroying and Altering Relations v Destroys the relation Students. The schema information and the tuples are deleted. DROP TABLE Students v 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. ALTER TABLE Students ADD COLUMN firstYear: integer

25. Database Management Systems 25 Raghu Ramakrishnan Adding and Deleting Tuples v Can insert a single tuple using: INSERT INTO Students (sid, name, login, age, gpa) VALUES (53699, 'Green ', 'green@ee', 18, 3.5) v More inserts: INSERT INTO Students (sid, name, login, age, gpa) VALUES (53666, 'Jones', 'jones@cs', 18, 3.4) INSERT INTO Students (sid, name, login, age, gpa) VALUES (53688, 'Smith ', 'smith@eecs', 18, 3.2) INSERT INTO Students (sid, name, login, age, gpa) VALUES (53650, 'Smith ', 'smith@math', 18, 3.5)

26. Database Management Systems 26 Raghu Ramakrishnan v Students relation after inserts: sidnameloginagegpa 53666Jonesjones@cs183.4 53688Smithsmith@eecs183.2 53650Smithsmith@math193.8 53600Greengreen@ee183.5 v Students relation before inserts: v Question: what happens if you insert the same info at a second time?

27. Database Management Systems 27 Raghu Ramakrishnan v Can delete all tuples satisfying some condition (e.g., name = Smith): DELETE FROM Students S WHERE S.name = 'Smith' sidnameloginagegpa 53666Jonesjones@cs183.4 53600Greengreen@ee183.5 v Students instance after delete: sidnameloginagegpa 53666Jonesjones@cs183.4 53688Smithsmith@eecs183.2 53650Smithsmith@math193.8 53600Greengreen@ee183.5

28. Database Management Systems 28 Raghu Ramakrishnan Adding and Deleting Tuples (continued) v Insert a tuples into the Bank_Acct instance: INSERT INTO Bank_Acct (acct, name, address, balance) VALUES (1025, 'Blue', '14 Third St', 2344.22) INSERT INTO Bank_Acct (acct, name, address, balance) VALUES (1005, 'Jones', '11 First St', 1023.22) INSERT INTO Bank_Acct (acct, name, address, balance) VALUES (1002, 'Smith', '12 First St', 22.43) INSERT INTO Bank_Acct (acct, name, address, balance) VALUES (1003, 'Smith', '12 First St', 11000.12) INSERT INTO Bank_Acct (acct, name, address, balance) VALUES (1008, 'Green', '14 Fourth St', 1077.23) INSERT INTO Bank_Acct (acct, name, address, balance) VALUES (1010, 'Gates', '20 Tenth St', 5000000000.00)

29. Database Management Systems 29 Raghu Ramakrishnan Adding and Deleting Tuples (continued) v Bank_Acct instance after insert: AcctName Balance 1005Jones $1023.22 1002Smith $22.43 1003Smith $11000.12 1008Green$1077.23 Address 11 First St 12 First St 14 Fourth St 1010Gates $5,000,000,000.00 20 Tenth St 1025Blue $2344.22 14 Third St

30. Database Management Systems 30 Raghu Ramakrishnan Adding and Deleting Tuples (continued) v Delete all tuples satisfying some condition (e.g., acct = 1008) from Bank_Acct instance: DELETE FROM Bank_Acct B WHERE B.acct = 1008

31. Database Management Systems 31 Raghu Ramakrishnan Adding and Deleting Tuples (continued) v Bank_Acct instance after delete: * Powerful variants of these commands are available; more later! AcctName Balance 1005Jones $1023.22 1002Smith $22.43 1003Smith $11000.12 Address 11 First St 12 First St 1010Gates $5,000,000,000.00 20 Tenth St 1012Smith $443.77 12 Second St 1025Blue $2344.22 14 Third St

32. Database Management Systems 32 Raghu Ramakrishnan The SQL Query Language v 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 from Students S

33. Database Management Systems 33 Raghu Ramakrishnan The SQL Query Language (continued) v To find all bank customers whose name is Smith : SELECT * FROM Bank_Acct B WHERE B.name='Smith' Name PhoneAddressEmail Smith 603333556712 First StSmith@cs Smith 6039788765 12 Second St Smith@ee

34. Database Management Systems 34 Raghu Ramakrishnan Adding and Deleting Tuples (continued) v Insert tuples into the Enrolled instance: INSERT INTO Enrolled (sid, cid, grade) VALUES ('53831', 'Carnatic 101', 'C') INSERT INTO Enrolled (sid, cid, grade) VALUES ('53831', 'Reggae 203', 'B') INSERT INTO Enrolled (sid, cid, grade) VALUES ('53650', 'Topology 112', 'A') INSERT INTO Enrolled (sid, cid, grade) VALUES ('53666', 'History 105', 'B')

35. Database Management Systems 35 Raghu Ramakrishnan Querying Multiple Relations v What does the following query compute? SELECT S.name, E.cid FROM Students S, Enrolled E WHERE S.sid=E.sid AND E.grade='B' S.name E.cid Jones History 105 we get: sidnameloginagegpa 53666Jonesjones@cs183.4 53688Smithsmith@eecs183.2 53650Smithsmith@math193.8 53600Greengreen@ee183.5

36. Database Management Systems 36 Raghu Ramakrishnan Querying Multiple Relations (continued) v What does the following query compute? SELECT B.name, C.email FROM Bank_Acct B, Customer_Info C WHERE B.name=C.name AND B.addr=C.addr AND B.balance>$1000.00 v Given the following instances of Bank_Acct and Customer_Info

37. Database Management Systems 37 Raghu Ramakrishnan Querying Multiple Relations (continued) Name Phone Jones 6032223456 Smith 6033335567 Address 11 First St 12 First St Green781444789014 Fourth St Gates 1223336789 11 First St Smith 6039788765 12 Second St Email Jones@cs Smith@cs Green@cs Gates@ms Smith@ee AcctName Balance 1005Jones$1023.22 1002Smith $22.43 1003Smith $11000.12 1008Green$1077.23 Address 11 First St 12 First St 14 Fourth St 1010Gates $5,000,000,000.00 20 Tenth St 1012Smith $443.77 15 Second St

38. Database Management Systems 38 Raghu Ramakrishnan Querying Multiple Relations (continued) v We get : Name Jones Smith Green Email Jones@cs Smith@cs Green@cs

39. Database Management Systems 39 Raghu Ramakrishnan Views v 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 v Views can be dropped using the DROP VIEW command. u How to handle DROP TABLE if there’s a view on the table?  DROP TABLE command has options to let the user specify this.

40. Database Management Systems 40 Raghu Ramakrishnan Views and Security v 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.

41. Database Management Systems 41 Raghu Ramakrishnan Logical DB Design: ER to Relational v Entity sets to tables. CREATE TABLE Employees (ssn CHAR (11), name CHAR (20), lot INTEGER, PRIMARY KEY (ssn)) Employees ssn name lot sssnamelot 312-24-234 John18 324-56-678 Jack2

42. Database Management Systems 42 Raghu Ramakrishnan lot dname budget did since name Works_In Departments Employees ssn Relationship Set Relationship Sets to Tables

43. Database Management Systems 43 Raghu Ramakrishnan Relationship Sets to Tables v In translating a relationship set to a relation, attributes of the relation must include: – Keys for each participating entity set (as foreign keys). u This set of attributes forms a superkey for the relation. – 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)

44. Database Management Systems 44 Raghu Ramakrishnan Entity Sets and their Relationship Set CREATE TABLE Employees (ssn CHAR (11), name CHAR (20), lot INTEGER, PRIMARY KEY (ssn)) CREATE TABLE Departments (did CHAR (11), dname CHAR (20), budget INTEGER, PRIMARY KEY (did)) 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)

45. Database Management Systems 45 Raghu Ramakrishnan lot dname budget did since name Works_In Departments Employees ssn Locations address capacity CREATE TABLE Works_In( ssn CHAR (11), did INTEGER, address CHAR(20) since DATE, PRIMARY KEY (ssn, did, address), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments, FOREIGN KEY (address) REFERENCES Locations)

46. Database Management Systems 46 Raghu Ramakrishnan CREATE TABLE Works_In( ssn CHAR (11), did INTEGER, address CHAR(20) since DATE, PRIMARY KEY (ssn, did, address), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments) FOREIGN KEY (address) REFERENCES Locations) CREATE TABLE Employees (ssn CHAR (11), name CHAR (20), lot INTEGER, PRIMARY KEY (ssn)) CREATE TABLE Departments (did CHAR (11), dname CHAR (20), budget INTEGER, PRIMARY KEY (did)) CREATE TABLE Locations (address CHAR (11), capacity CHAR (20), PRIMARY KEY (address))

47. Database Management Systems 47 Raghu Ramakrishnan Describe Key Constraints v Each dept has at most one manager, according to the key constraint on Manages. dname budget did since lot name ssn Manages Employees Departments lot dname budget did since name Works_In Departments Employees ssn Relationship Set key constraint

48. Database Management Systems 48 Raghu Ramakrishnan Translating ER Diagrams with Key Constraints v Map relationship to a table: – Note that did is the key now! – Separate tables for Employees 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) dname budget did since lot name ssn Manages Employees Departments

49. Database Management Systems 49 Raghu Ramakrishnan dname budget did since lot name ssn Manages Employees 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 Departments (did CHAR (11), dname CHAR (20), budget INTEGER, PRIMARY KEY (did)) CREATE TABLE Employees (ssn CHAR (11), name CHAR (20), lot INTEGER, PRIMARY KEY (ssn))

50. Database Management Systems 50 Raghu Ramakrishnan Translating ER Diagrams with Key Constraints Since each department has a unique manager, we could instead combine Manages and 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) dname budget did since lot name ssn Manages Employees Departments

51. Database Management Systems 51 Raghu Ramakrishnan dname budget did since lot name ssn Manages Employees 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) CREATE TABLE Employees (ssn CHAR (11), name CHAR (20), lot INTEGER, PRIMARY KEY (ssn))

52. Database Management Systems 52 Raghu Ramakrishnan lot dname budget did since name Works_In Departments Employees ssn Can we do like this? Each object in database should have a primary key!

53. Database Management Systems 53 Raghu Ramakrishnan Describe Participation Constraints v Does every department have a manager? – If so, this is a participation constraint : the participation of Departments in Manages is said to be total (vs. partial ). u Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!) lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In

54. Database Management Systems 54 Raghu Ramakrishnan Participation Constraints in SQL v 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 )

55. Database Management Systems 55 Raghu Ramakrishnan CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees) 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 ) Different? Why?

56. Database Management Systems 56 Raghu Ramakrishnan lot name dname budgetdid since name dname budgetdid since Manages Departments Employees ssn dname budget did since lot name ssn Manages Employees Departments Different? Some departments may not have manager currently! Each department should always have manager!

57. Database Management Systems 57 Raghu Ramakrishnan Describe Weak Entities v A weak entity can be identified uniquely only by considering the primary key of another ( owner ) entity. – Owner entity set and weak entity set must participate in a one-to-many relationship set (1 owner, many weak entities). – Weak entity set must have total participation in this identifying relationship set. lot name age pname Dependents Employees ssn Policy cost

58. Database Management Systems 58 Raghu Ramakrishnan Translating Weak Entity Sets v 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 )

59. Database Management Systems 59 Raghu Ramakrishnan Binary vs. Ternary Relationships v If each policy is owned by just 1 employee: – Key constraint on Policies would mean policy can only cover 1 dependent! v What are the additional constraints in the 2nd diagram? age pname Dependents Covers name Employees ssn lot Policies policyid cost Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Bad design Better design

60. Database Management Systems 60 Raghu Ramakrishnan Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Why? Review: Binary vs. Ternary Relationships

61. Database Management Systems 61 Raghu Ramakrishnan Binary vs. Ternary Relationships (Contd.) v The key constraints allow us to combine Purchaser with Policies and Beneficiary with Dependents. v Participation constraints lead to NOT NULL constraints. CREATE TABLE Policies ( policyid INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (policyid). FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE ) CREATE TABLE Dependents ( pname CHAR(20), age INTEGER, policyid INTEGER, PRIMARY KEY (pname, policyid). FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE )

62. Database Management Systems 62 Raghu Ramakrishnan lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In age pname DependentsPolicy cost

63. Database Management Systems 63 Raghu Ramakrishnan Relational Model: Summary v A tabular representation of data. v Simple and intuitive, currently the most widely used. v 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. v Powerful and natural query languages exist. v Rules to translate ER to relational model

64. Database Management Systems 64 Raghu Ramakrishnan https://servlets.uncc.edu/oracle-account/ How to obtain your Oracle account?