ER to REL

Mapping E/R Diagrams toRelational Database Schemas: 

Mapping E/R Diagrams to Relational Database Schemas Second Half of Chapter 3

E/R  Relation Model (Example): 

E/R  Relation Model (Example) lot name dname budget did since name dname budget did since Manages since Departments Employees ssn Works_In (0,*) (0,*) (0,*) (1,1) Contract_Emps hourly_wages Hourly_Emps contractid hours_worked  

Dr. Eick’s Default MappingE/R  Relational Data Model: 

Dr. Eick’s Default Mapping E/R  Relational Data Model For each entity type create a relation with the attributes associated with the entity type. Choose a primary key for the defined relation; if the entity type is weak, delay choosing primary keys until all identifying relationships are mapped. For each relationship type create a relation that contains the roles as well as the attributes of the relationship type. Define referential integrity constraints with respect to the mapped roles. Exception: If there is a (1,1) cardinality constraint do not generate a separate relation, but rather associate the relationship information with the relation of this participating entity type. For each sub-type create a relation that contains the attributes of the entity type as well as the primary key of the most general super class of this entity type (which also will be the primary key of the generated relation). Define referential integrity constraints with respect to the direct super class of the mapped entity type.

Logical DB Design: ER to Relational: 

Logical DB Design: ER to Relational 1. Entity Types to Tables. CREATE TABLE Employees (ssn CHAR(9), name CHAR(20), lot INTEGER, PRIMARY KEY (ssn))

2. Relationship Types to Tables: 

2. Relationship Types to Tables In translating a relationship set to a relation, attributes of the relation must include: Keys for each participating entity set (as foreign keys). This set of attributes forms a superkey for the relation. All descriptive attributes. CREATE TABLE Works_In( ssn CHAR(9), did INTEGER, since DATE, PRIMARY KEY (ssn, did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments)

Translating ER Diagrams with Key Constraints: 

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(9), 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, manager CHAR(9), since DATE, PRIMARY KEY (did), FOREIGN KEY (manager) REFERENCES Employees)

Review: Weak Entities: 

Review: Weak Entities 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 dname Dependents Employees ssn Policy cost Dependents Policy (1,1) (0,*) Parent

Translating Weak Entity Types: 

Translating Weak Entity Types Weak entity set and identifying relationship set are translated into a single table --- it has a (1,1) cardinality constraint. CREATE TABLE Dep_Policy ( dname CHAR(20), age INTEGER, cost REAL, parent CHAR(9) NOT NULL, PRIMARY KEY (pname, ssn), FOREIGN KEY (parent) REFERENCES Employees, ON DELETE CASCADE)

Review: ISA Hierarchies: 

Review: ISA Hierarchies Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked As in C++, or other PLs, attributes are inherited. If we declare A ISA B, every A entity is also considered to be a B entity. 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)

3. Translating ISA Hierarchies to Tables: 

3. Translating ISA Hierarchies to Tables General approach: 3 relations: Employees, Hourly_Emps and Contract_Emps. 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). 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.

Dr. Eick’s Default MappingE/R  Relational Data Model: 

Dr. Eick’s Default Mapping E/R  Relational Data Model For each entity type create a relation with the attributes associated with the entity type. Choose a primary key for the defined relation; if the entity type is weak, delay choosing primary keys until all identifying relationships are mapped. For each relationship type create a relation that contains the roles as well as the attributes of the relationship type. Define referential integrity constraints with respect to the mapped roles. Exception: If there is a (1,1) cardinality constraint do not generate a separate relation, but rather associate the relationship information with the relation of this participating entity type. For each sub-type create a relation that contains the attributes of the entity type as well as the primary key of the most general super class of this entity type (which also will be the primary key of the generated relation). Define referential integrity constraints with respect to the direct super class of the mapped entity type.

Slide12: 

Male Female Person name ssn Wedding (0,*) E/R Diagram to be mapped occurred (0,*) from to (1,1) Is-insured Company name amount (0,*) (0,*) location  husband wife Con#

Mapping of the Multi-Wedding E/R Diagram to a Relational Schema: 

Mapping of the Multi-Wedding E/R Diagram to a Relational Schema Person(ssn,name) Company(name,location) Male_Person(ssn) Female_Person(ssn) Wedding(husband,wife,from,to) Is-Insured(hssn,wssn,from,company, amount, Con#) Correct Syntax: FOREIGN KEY (hssn,wssn,from) REFERENCES Wedding(husband,wife,from)