Transforming an ER Model into a Relational Schema  Cs263 Lecture 10

Transforming an ER Model into a Relational Schema There are a series of steps that are recommended for converting an E/ER model into a relational schema. These steps are not hard and fast rules, but rather a series of pertinent suggestions that the database designer must use wisely.

E/ER to Relational Schema Conversion Steps Step 1  For each ‘strong’ entity in the E/ER model, create a base relation with a column for each simple attribute of that entity.  The key attribute for the entity becomes the primary key of the relation.

Example – Converting ‘Strong’ Entities Emp EmpNo Salary Comm Hire Date Name Job EmpNoNumber(4) NameVarchar2(10) JobVarchar2(9) SalNumber(7,2) CommNumber(7,2) HireDateDate Emp Primary Key

Example – Converting ‘Strong’ Entities Dept DeptNo Location Name DeptNoNumber(2) NameVarchar2(14) LocationVarchar2(13) Dept Primary Key

E/ER to Relational Schema Conversion Steps Step 2 For each ‘weak’ entity, create a relation consisting of all the simple attributes of that entity and also include columns for the primary keys of those entities on whose existence it is dependent.

Example – Converting ‘Weak’ Entities Loan LoanNoChar(8) PaymentNoNumber(4)Date AmountNumber(9,2) Payment Composite Primary Key Payment Date Amount Loan No Payment No Discriminator

E/ER to Relational Schema Conversion Steps Step 3 When two entities participate in a one-to- many (1-M) relationship, the relation representing the entity with the M (Many) cardinality must have a foreign key column representing this relationship.

Example – Converting (1-M) Relationships Dept D# Emp Works in Assigned E# EmpNoNumber(4) NameVarchar2(10) JobVarchar2(9) SalNumber(7,2) CommNumber(7,2) HireDateDate DeptNoNumber(2) Emp Foreign Key As an Employee ‘must’ work in a department, then the DeptNo column in the Emp relation (table) cannot be NULL!

Example – Converting (1-M) Relationships EmpNoNumber(4) NameVarchar2(10) JobVarchar2(9) SalNumber(7,2) CommNumber(7,2) HireDateDate DeptNoNumber(2) Emp Foreign Key As an Employee ‘may not’ work in a department, then the DeptNo column in the Emp relation (table) can be NULL! Dept D# Emp Works in Assigned E#

E/ER to Relational Schema Conversion Steps Step 4 When two entities participate in a (1-1) relationship, a foreign key column must be included in the relation that represents one of these entities.

Example – Converting (1-1) Relationships Emp E# ID Card Belongs to Has ID# EmpNoNumber(6) NameVarchar2(10) Address Varchar2(200 ) DOBDate IDNoChar(10) Emp Foreign Key IDNoChar(10) IssuedDate ExpiresDate EmpNoNumber(6) ID Card Foreign Key EITHER ONE IS ACCEPTABLE

E/ER to Relational Schema Conversion Steps Step 5 When two entities participate in a many-to- many (M-M) relationship, then a relation must be created consisting of foreign keys for the two relations that represent the participating entities.

Example – Converting (M-N) Relationships Person P# Car Driven by Drives C# PersonNoNumber(6) CarNoChar(7) Person-Car Foreign Key Person P# Car C# Person-Car P#,C#

E/ER to Relational Schema Conversion Steps Step 6 Where an entity has a multi-valued attribute, create a relation with a column as a foreign key to the relation that represents the entity and a column for the multi-valued attribute.

Language Example – Multi-valued Attributes Programmer EmpNo Language Name EmpNoNumber(2) NameVarchar2(14) Programmer Primary Key EmpNoNumber(2) LanguageVarchar2(20) Prog-Lang Composite Primary Key

E/ER to Relational Schema Conversion Steps Step 7 When more than two entities participate in a relationship, then a relation must be created consisting of foreign keys to those relations representing the entities participating in the relationship.

Example – Converting Ternary Relationships Match Match No Player Player No Offence Offence No Booking MatchNo Number(4) PlayerNo Number(6) OffenceNo Number(3) Time Date Booking Composite Primary Key

E/ER to Relational Schema Conversion Steps Step 8 – Option A For sub-typing, create separate relations for each subtype consisting of those attributes which are peculiar to the given subtype, and include a foreign key to the super-type, that will act as the sub-type’s primary key. Create a separate relation for the super- type!

Example – Converting Sub-types EmployeeN o Name Address Employee EMPLOYEE Employee No Name Address SECRETARY Typing Speed TECHNICIAN Grade ENGINEER Type EmployeeN o TypingSpee d Secretary EmployeeN o Grade Technician EmployeeN o Type Engineer

E/ER to Relational Schema Conversion Steps Step 8 – Option B For sub-typing, create separate relations for each subtype consisting of all the attributes of the super-type, and all those which are peculiar to the given subtype. The primary key of the super-type becomes the primary key of each sub-type. Do not create a relation for the super-type!

Example – Converting Sub-types VehicleNo LicenceNo Price NoOfSeats Car VEHICLE Vehicle No Licence No Price CAR No of Seats TRUCK No of Axles Tonnage Truck VehicleNo LicenceNo Price NoOfAxles Tonnage