ER to Relational Mapping

Transcription

1 ER to Relational Mapping ER Models Used in Conceptual Modeling of the Database Schema Meant for Human Comprehension High-level database design without implementation details DBMS independent Building Blocks: Entities, Attributes, Relationships 1

2 Relational Models Data model for Physical Schema Design Meant for machine consumption DBMS independent; however many DBMS specific features used in practice Automated optimization steps based on functional dependencies Building blocks: Relations, attributes, keys Some ER Notations Employee Entity Type Insurance Record Weak Entity Type Handles Relationship Type Insurance Details Identifying Relationship Type 2

3 Some ER Notations Attribute PAN Key Attribute Multi-valued Attribute Derived Attribute Some Definitions A data value is said to be atomic if it cannot be subdivided into smaller data values. For example, the age of a person in years is an atomic value. A Domain is a set of atomic values. Examples are: the set of all Integers, the set of all valid student roll number, the set of all Indian cities with population above 6 million etc. A Relation Schema denoted by R(A 1,A 2,A 3,A n ) is made up of relation name R and a list of attributes A 1,A 2,A 3,A n. Each A i is the name of role played some domain D in the schema R. The domain A i is denoted by dom(a i ). 3

4 Some Definitions The Degree of the relation is the number of attributes of its relation schema. A relation r of a relation schema R(A 1,A 2,A 3,A n ), is a set of n-tuples of the form [t 1,t 2,.,t n ] where each t i dom(a i ). The Relation can be defined as r(r) (dom(a 1 ) x dom(a 2 ) x dom(a n )), where x is the cartesian product over sets. Relational Key Constraints Key constraints: Given a relational schema R = (A i ), i = 1 n, a subset of attributes K is called a superkey if the values of attributes in K is distinct for every relation instance. This is, for any two tuples t i, t j, (t i t j ) (t i [K] t j [K]). A key k of a relation is a subset of the superkey, k K,such that removal of any attribute in k removes the superkey property for k. They are also called Minimal superkeys. Entity Integrity Constraint: The primary key of a tuple may never be null. 4

5 Relational Constraints Referential Integrity Constraint: Informally, a tuple that refers to another tuple from another relation should refer to an existing tuple. In a given relation R 1 a set of attributes FK is said to be a foreign key of R 1 referencing another relation R 2, if the following rules hold: 1. The attribute in FK have the same domain as the primary key of R For every tuple in R 1, the attributes in its Foreign Key FK either reference a tuple in R 2 or is null. Relations as Tables Relation: Student (RollNo, Name, Lab) Roll No Name Ahmed Riaz Humanyun Aslam Lab Speech Lab Open system Lab Distributed computing lab Open system lab 5

6 ER to Relational Mapping Rules for automated translation from ER to Relational models help in building complete lifecycle tools. Example: ERWin from Computer Associates ER to Relational Mapping Dept Name Dept_Id Manager Department Department (Dept_id, DeptName, Manager) Strong Entity Types with simple attributes can be mapped into relations in a straightforward manner. Attributes acting as keys in the ER model are retained as keys in the relational model. 6

7 ER to Relational Mapping Loc_id RGN_NO Dept Name Dept_Id Manager Department Department (Loc_id, RGN_NO, DeptName, Manager) All simple components of composite attributes can be added to the set of attributes in the formed relation. Keys are retained in a natural manner. ER to Relational Mapping PAN Employee Insurance 1 Details 1 Insurance Record InsuranceRecord (PAN, ) For weak entity types, include the key of the owner entity type as key attribute(s). These key attributes also form a foreign key into the owner entity type. The CASCADE option is used whenever the owner entity type is updated or deleted. 7

8 Mapping 1:1 Relationships Dept_id Secretary EmpNo Department ManagedBy 1 1 Manager Manager (EmpNo, Dept_id, Secretary, ) Mapping 1:1 Relationships In a 1:1 binary relationship between two entity types S and T, choose one of them (say S) as the base relation. If either S or T has a total participation, choose that one as the base. Include the primary key of the other entity type as a foreign key of the base. Include any relationship attributes as attributes of the base. 8

9 Mapping 1:1 Relationships Proj_id Secretary PAN Project Consultation 1 1 Consultant Consultation (Proj_id, PAN, Secretary, ) Mapping 1:1 Relationships In case both entities in a 1:1 binary relationship are in total participation, then merge both entity types into one (usually in the name of the relationship). 9

10 Mapping 1:N Relationships EmpNo Dept_id Employee WorksIn N 1 Department Employee (EmpNo, Dept_id, ) Mapping 1:N Relationships For each binary 1:N relationships, identify the relation S that represents the entity type on the N side. Use this as the base and create a relation including the key of the other entity type as a foreign key. (Each entity type on the N side is related to at most one entity on the 1 side). 10

11 Mapping M:N Relationships EmpNo RecordNo Dept_id Employee M DeputedTo N Department Employee (EmpNo, ) Department (Dept_id, ) DeputedTo (EmpNo, Dept_id, RecordNo, ) Mapping M:N Relationships In a M:N binary relationship, it is not possible to collapse the relationship into any one of the entity types, since the relationship does not identify either entity type uniquely. A separate relation is required to complete the mapping. The CASCADE option on referential integrity should be used for the relation created during UPDATES and DELETES. This is because, each instance of the relationship has an existential dependency on the created relation. 11

12 Mapping M:N Relationships Both 1:1 and 1:N relationships can be mapped in a way similar to mapping M:N relationships. This is especially true if number of relationship instances are small and null-values have to be avoided in the created relation. However, this is not attractive if there is total participation in 1:N or 1:1 relationships due to the creation of extra unnecessary relations. Mapping Multi-valued Attributes Species Colour Bird Bird (Species, ) BirdColours (Species, Colour) 12

13 Mapping Multi-valued Attributes For each multi-valued attributes of a given entity type S, create a separate relation that has the primary key of S, paired with all values of the multi-valued attribute. The CASCADE option should be used for referential integrity on the newly created relation. Mapping n-ary Relationships STRegNo ProjId Supplier Supplies Project Part PartId Supplier (STRegNo, ) Project (ProjId, ) Part (PartId, ) Supplies (STRegNo, ProjId, PartId) 13

14 Mapping n-ary Relationships For each n-ary relationship type R, where n>2, create a new relation S to represent R. Primary keys of participating relations in R become foreign keys in S. CASCADE option should be used for updates and deletes. Summary of Mappings ER Model Entity type 1:1 or 1:N relationship type M:N relationship type Relational Model Entity Relation Foreign key from N-side to 1-side Relationship relation with two foreign keys 14

15 Summary of Mappings ER Model N-ary relationship type Simple attribute Relational Model Relationship relation with n foreign keys Attribute Composite attribute Set of simple attributes Summary of Mappings ER Model Multi-valued attribute Value Set Relational Model Relation and foreign key Domain Key attribute Primary or secondary key 15