Michael F. Price College of Business Chapter 6: Logical

Michael F. Price College of Business Chapter 6: Logical database design and the relational model

Michael F. Price College of Business Objectives of logical design. Translate the conceptual design into a logical database design that can be implemented on a chosen DBMS Input: conceptual model (ERD) Output: relational schema, normalized relations Resulting database must meet user needs for: Data sharing Ease of access Flexibility

Michael F. Price College of Business Relational database components Data structure Data organized into tables Data manipulation Add, delete, modify, and retrieve using SQL Data integrity Maintained using business rules

Michael F. Price College of Business Why do I need to know this? Mapping conceptual models to relational schema is straight-forward CASE tools can perform many of the steps, but. Often CASE cannot model complexity of data and relationship (e.G., Ternary relationships, supertype/subtypes) There are times when legitimate alternates must be evaluated You must be able to perform a quality check on CASE tool results

Michael F. Price College of Business Some rules. Every table has a unique name. Attributes in tables have unique names. Every attribute value is atomic. Multi-valued and composite attributes? Every row is unique. The order of the columns is irrelevant. The order of the rows is irrelevant.

Michael F. Price College of Business The key. Relational modeling uses primary keys and foreign keys to maintain relationships Primary keys are typically the unique identifier noted on the conceptual model Foreign keys are the primary key of another entity to which an entity has a relationship Composite keys are primary keys that are made of more than one attribute Weak entities Associative entities

Michael F. Price College of Business Implementing it Attribute Instance Field Entity

Michael F. Price College of Business What about relationships?

Michael F. Price College of Business Constraints Constraints Domain constraints Allowable values for an attribute as defined in the domain Entity integrity constraints No primary key attribute may be null Operational constraints Business rules Referential integrity constraints

Michael F. Price College of Business Referential Referential integrity integrity constraint constraint Maintains consistency among rows of two entities matching of primary and foreign keys Enforcement options for deleting instances Restrict Cascade Set-to-Null

Michael F. Price College of Business Transforming Transforming the the EER EER diagram diagram into relations The steps: Map regular entities Map weak entities Map binary relationships Map associative entities Map unary relationships Map ternary relationships Map supertype/subtype relationships

Michael F. Price College of Business Transforming E-R diagrams into relations Mapping regular entities to relations Composite attributes: use only their simple, component attributes Multi-valued attributes: become a separate relation with a foreign key taken from the superior entity

Michael F. Price College of Business Mapping a composite attribute

Michael F. Price College of Business Looks like this using relational schema schema notation notation

Michael F. Price College of Business Transforming E-R diagrams into relations Mapping weak entities Becomes a separate relation with a foreign key taken from the superior entity

Michael F. Price College of Business Example of mapping a weak entity

Michael F. Price College of Business Looks like this using relational schema schema notation notation

Michael F. Price College of Business Transforming E-R diagrams into relations Mapping binary relationships One-to-many - primary key on the one side becomes a foreign key on the many side Many-to-many - create a new relation (associative entity) with the primary keys of the two entities as its primary key – I like to call these intersection entities to distinguish them from associative entities created at the conceptual level One-to-one - primary key on the mandatory side becomes a foreign key on the optional side

Michael F. Price College of Business Example Example of of mapping mapping aa 1:M 1:M relationship

Michael F. Price College of Business Looks like this using relational schema schema notation notation

Michael F. Price College of Business Example of mapping an M:M relationship

Michael F. Price College of Business Looks like this using relational schema schema notation notation

Michael F. Price College of Business Mapping a binary 1:1 relationship

Michael F. Price College of Business Looks like this using relational schema schema notation notation

Michael F. Price College of Business Transforming E-R diagrams into relations Mapping associative entities Identifier not assigned – Default primary key for the association relation is the primary keys of the two entities Identifier assigned – It is natural and familiar to end-users – Default identifier may not be unique

Michael F. Price College of Business Mapping an associative entity with an identifier

Michael F. Price College of Business Looks like this using relational schema schema notation notation

Michael F. Price College of Business Transforming E-R diagrams into relations Mapping unary relationships One-to-many - recursive foreign key in the same relation Many-to-many - two relations: – One for the entity type – One for an associative relation in which the primary key has two attributes, both taken from the primary key of the entity

Michael F. Price College of Business For example. Emp Num EMPLOYEE Emp-Name Emp Address Supervises

Michael F. Price College of Business Would look like. references Emp Num Emp Name Emp Address Boss Num

Michael F. Price College of Business And. Num Units Comp Num COMPONENT Description Unit of-Measure BOM

Michael F. Price College of Business Would look like. COMPONENT Comp Num Desc Unit of Measure BOM Num-of Units Comp Num Subassembly Num

Michael F. Price College of Business Transforming E-R diagrams into relations Mapping ternary (and n-ary) relationships One relation for each entity and one for the associative entity

Michael F. Price College of Business Mapping Mapping aa ternary ternary relationship

Michael F. Price College of Business Looks like this using relational schema schema notation notation

Michael F. Price College of Business Transforming E-R diagrams into relations Mapping Supertype/subtype relationships Create a separate relation for the supertype and each of the subtypes Assign common attributes to supertype Assign primary key and unique attributes to each subtype Assign an attribute of the supertype to act as subtype discriminator

Michael F. Price College of Business Mapping Supertype/subtype relationships

Michael F. Price College of Business Would look like this.

Michael F. Price College of Business Let’s try a couple .

Michael F. Price College of Business Well-structured relations Well-structured relations contain minimal redundancy and allow insertion, modification, and deletion without errors or inconsistencies Anomalies are errors or inconsistencies resulting from redundancy Insertion anomaly Deletion anomaly Modification anomaly

Michael F. Price College of Business Data normalization Normalization is a formal process for deciding which attributes should be grouped together in a relation Objective: to validate and improve a logical design so that it satisfies certain constraints that avoid unnecessary duplication of data Definition: the process of decomposing relations with anomalies to produce smaller, well-structured relations

Michael F. Price College of Business Steps in normalization

Michael F. Price College of Business Functional dependencies and keys Functional dependency: the value of one attribute (the determinant) determines the value of another attribute A - B, for every valid instance of A, that value of A uniquely determines the value of B Candidate key: an attribute or combination of attributes that uniquely identifies an instance Uniqueness: each non-key field is functionally dependent on every candidate key Non-redundancy

Michael F. Price College of Business First First normal normal form form No multi-valued attributes. Every attribute value is atomic.

Michael F. Price College of Business Second Second normal normal form form 1NF and every non-key attribute is fully functionally dependent on the primary key. Every non-key attribute must be defined by the entire key, not by only part of the key. No partial functional dependencies.

Michael F. Price College of Business Third normal form 2NF and no transitive dependencies (functional dependency between non-key attributes.)

Michael F. Price College of Business Relation with transitive dependency

Michael F. Price College of Business Transitive dependency in SALES relation

Michael F. Price College of Business Removing a transitive dependency

Michael F. Price College of Business Relations in 3NF

Michael F. Price College of Business Let’s practice.

Michael F. Price College of Business Other considerations. Synonyms: different names, same meaning. Homonyms: same name, different meanings.