Book Chapter 3 (part 2 )

1. From ER to Relational Model Book Chapter 3 (part 2 )

2. name ssn lot Employees Logical DB Design: ER to Relational • Translate Entity sets to tables: CREATE TABLE Employees (ssn CHAR(11), name CHAR(20), lot INTEGER, PRIMARY KEY (ssn))

3. Translate Relationship Sets to Tables since name dname ssn budget lot did Works_In Employees Departments

4. Translate Relationship Sets to Tables CREATE TABLE Works_In( ssn CHAR(11), did INTEGER, since DATE) • Attributes of relation include: • Keys for each participating entity set • All descriptive attributes. since name dname ssn budget lot did Works_In Employees Departments

5. Translate Relationship Sets to Tables CREATE TABLE Works_In( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (ssn, did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments) • Foreign Keys: • Keys for each participating entity set (?) • Keys: • This set of attributes forms superkey for relation (?) since name dname ssn budget lot did Works_In Employees Departments

6. Key Constraints Translation to relational model? 1-to-1 1-to Many Many-to-1 Many-to-Many

7. since name dname ssn lot Employees Manages Review: Key Constraint • Each dept has at most one manager, according to key constrainton Manages. • Each department appears only once in relationship budget did Departments Translation to relational model? 1-to Many

8. Translate Key Constraint : Approach I. • Separate tables for Employees and Departments. • Note that did is key now! TABLE Dept(…) TABLE Employee (…) CREATE TABLE Manages( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments)

9. since name dname ssn lot Employees Manages Translate Key Constraint: Approach II. TABLE Employee (…) CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, ………………. ) • Combine Manages and Departments into one relation. • Each department has a unique manager budget did Departments

10. since name dname ssn lot Employees Manages Translate Key Constraint: Approach II. TABLE Employee (…) CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees) • Combine Manages and Departments into one relation. • Each department has a unique manager, if any. budget did Departments

11. Other Cases of Key Constraints 1-to-1 1-to Many Many-to-1 Many-to-Many Translation to relational model?

12. Review: Participation Constraints • The “must have” constraint (not-null value)? since since name name dname dname ssn did did budget budget lot Departments Employees Manages Works_In since

13. Participation + Key Constraint. • Every department must have a manager ! • Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!) since since name name dname dname ssn did did budget budget lot Departments Employees Manages

14. Participation Constraints in SQL • Approach I. • every did value in Department appears in a tuple of Managers • corresponding tuple must have a non-null ssn values • Does this capture with “not-null” work, or not? TABLE Dept(…) TABLE Employee (…) CREATE TABLE Manages( ssn CHAR(11) NOT NULL, did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments,

15. Participation Constraints in SQL • Approach I. • every did value in Department appears in a tuple of Works_In • the corresponding tuple must have a non-null ssn values TABLE Dept_mgr(…) TABLE Employee (…) CREATE TABLE Manages( ssn CHAR(11) NOT NULL, did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments, CHECK ( (SELECT Count(*) FROM Manages) = (SELECT Count(*) FROM Dept) ) • Must utilize check constraints !

16. Participation Constraints in SQL • Approach II. - capture participation constraints involving one entity set in a binary relationship using combined table. TABLE Employee (…) 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, SEMANTICS ??? ) Does this “non-null” approach now work ?

17. Participation Constraints in SQL • Approach II. - capture participation constraints involving one entity set in a binary relationship using combined table. TABLE Employee (…) 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, SEMANTICS ??? ) What should happen if manager-employee is deleted?

18. Participation Constraints in SQL • Approach II. - use the “on action” propagation constraint ! 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)

19. More on Participation Constraints since since • What about other cases of “must have” constraint ? name name dname dname ssn did did budget budget lot Departments Employees Manages Works_In since • What if we want to capture participation for many-to-many relationships? • Anwer: We need to use CHECK constraints.

20. Participation Constraints in SQL • What if we want to capture participation for three-way relationships? • Anwer: We need to use CHECK constraints. • Observation : Little else can be enforced without resorting to the use of check constraints !

21. Review: Weak Entities • A weak entity can be identified uniquely only by considering 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. name cost pname age ssn lot Policy Dependents Employees

22. Translating Weak Entity Sets • Weak entity set and identifying relationship set are translated into a single table. 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, WHAT SEMANTICS HERE ??? ) • What when the owner entity is deleted? • Then all owned weak entities must also be deleted !

23. Translating Weak Entity Sets • 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), PRIMARY KEY (pname, ssn), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE)

24. Review: ISA Hierarchies • Attributes are inherited name ssn lot Employees hours_worked hourly_wages ISA contractid Contract_Emps Hourly_Emps

25. Translating ISA Hierarchies • What are possible alternatives of mapping IS-A to the relational model ?

26. Translating ISA Hierarchies • General approach: • 3 relations: Employees, Hourly_EmpsandContract_Emps. Employees (ssn, name, lot) Hourly_Emps (ssn, hourly_wages, hours_worked);Contract_Emps (ssn, contractid); Keys? Foreign keys? Delete semantics? - Delete Hourly_Emps tuple if referenced Employees tuple is deleted - how? - Put CASCADE semantics on foreign key.

27. Translating ISA Hierarchies • Alternative: Two tables • Namely: Hourly_Emps and Contract_Emps Hourly_Emps (ssn, name, lot, hourly_wages, hours_worked) Contract_Emps (ssn, name, lot, contractid) Keys? Foreign keys? Delete semantics?

28. Pros/Cons : Translating ISA Hierarchies • Alternative : Three Relations Employees (ssn, name, lot) Hourly_Emps (ssn, hourly_wages, hours_worked);Contract_Emps (ssn, contractid); • Alternative: Two tables Hourly_Emps (ssn, name, lot, hourly_wages, hours_worked) Contract_Emps (ssn, name, lot, contractid) • Pros/cons for three relations: + Queries involving all employees easy - Queries involving just Hourly_Emps may require a Join. • Pros/Cons for two relations: • Each employee must be in one of these two subclasses. • Avoid joins for subtable queries

29. ISA Hierarchy Translation? name ssn lot • 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) Employees hours_worked hourly_wages ISA contractid Contract_Emps Hourly_Emps

30. name ssn lot Employees Beneficiary Policies policyid cost Another Example Requirements: • 1. A policy cannot be owned by two or more employees.(Key Constraints) • 2. Every policy must be owned by some employee (Total participation) • 3. Dependents is a weak entity (Weak Entity) pname age Dependents Purchaser

31. Example of Mapping to Tables CREATE TABLE Policies ( policyid INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (policyid). FOREIGN KEY (ssn) REFERENCES Employees) • Key constraints allow us to combine (Purchaser with Policies) and (Beneficiary with Dependents.) • Participation constraints lead to NOT NULL constraints. • Policies is a weak entity set: 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)

32. Summary • ER Modeling : graphical design view • Relational Model: A tabular representation of data. • Rules to translate ER to relational model exist • Later : Yet more design optimizations can and should be applied, after initial relational design has been derived.