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

2  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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)) Employees ssn name lot

3  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Translate Relationship Sets to Tables lot dname budget did since name Works_In DepartmentsEmployees ssn

4  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Translate Relationship Sets to Tables  Attributes of relation include:  Keys for each participating entity set  All descriptive attributes. CREATE TABLE Works_In( ssn CHAR(11), did INTEGER, since DATE) lot dname budget did since name Works_In DepartmentsEmployees ssn

5  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Translate Relationship Sets to Tables  Foreign Keys: Keys for each participating entity set (?)  Keys: This set of attributes forms superkey for relation (?) 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) lot dname budget did since name Works_In DepartmentsEmployees ssn

6  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Key Constraints Translation to relational model? Many-to-Many1-to-11-to ManyMany-to-1

7  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Review: Key Constraint  Each dept has at most one manager, according to key constraint on Manages.  Each department appears only once in relationship Translation to relational model? 1-to Many dname budget did since lot name ssn Manages Employees Departments

8  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Translate Key Constraint: Approach II.  Combine Manages and Departments into one relation.  Each department has a unique manager TABLE Employee (…) CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, ………………. ) dname budget did since lot name ssn Manages Employees Departments

10  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Translate Key Constraint: Approach II.  Combine Manages and Departments into one relation.  Each department has a unique manager, if any. 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) dname budget did since lot name ssn Manages Employees Departments

11  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Translate Key Constraints  Approach I  Separate tables for Employees and Departments.  Note that did is the key now!  Approach II  Combine Manages and Departments.  Each department has a unique manager 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) 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)

12  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Other Cases of Key Constraints Translation to relational model? Many-to-Many1-to-11-to ManyMany-to-1

13  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Review: Participation Constraints  The “must have” constraint (not-null value)? lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In

14  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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!) lot name dname budgetdid since name dname budgetdid since Manages Departments Employees ssn

15  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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,

16  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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 !

17  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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 ?

18  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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?

19  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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)

20  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational More on Participation Constraints  What about other cases of “must have” constraint ? lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In  What if we want to capture participation for many- to-many relationships?  Anwer: We need to use CHECK constraints.

21  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Participation Constraints in SQL  What if we want to capture participation for many-to- many relationships?  Anwer: We need to use CHECK constraints. lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In

22  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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 !

23  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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. lot name age pname Dependents Employees ssn Policy cost

24  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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 !

25  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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)

26  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Review: ISA Hierarchies Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked  Attributes are inherited

27  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Translating ISA Hierarchies  What are possible alternatives of mapping IS-A to the relational model ?

28  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Translating ISA Hierarchies  General approach:  3 relations: Employees, Hourly_Emps and Contract_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.

29  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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?

30  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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

31  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational ISA Hierarchy Translation? Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked  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)

32  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Translating ISA Hierarchies  General approach:  3 relations: Employees, Hourly_Emps and Contract_Emps. Employees ( ssn, name, lot) Hourly_Emps (ssn, hourly_wages, hours_worked) ; Contract_Emps (ssn, contractid); delete Hourly_Emps tuple if referenced Employees tuple is deleted (how?) + Queries involving all employees easy - Queries involving just Hourly_Emps require a Join.  Alternative: Just Hourly_Emps and Contract_Emps. Hourly_Emps (ssn, name, lot, hourly_wages, hours_worked) Hourly_Emps ( ssn, name, lot, contractid)  Each employee must be in one of these two subclasses.

33  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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) Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot

34  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational Example of Mapping to Tables  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 Policies ( policyid INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (policyid). FOREIGN KEY (ssn) REFERENCES Employees) CREATE TABLE Dependents ( pname CHAR(20), age INTEGER, policyid INTEGER, PRIMARY KEY (pname, policyid). FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE)

35  Relational Data Model  Relational Query Language  Integrity Constraints  ER to Relational 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.