Logical Database Design and the Relational Model
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Modern Database Management
11th Edition, International Edition
Jeffrey A. Hoffer, V. Ramesh,
Heikki Topi
Relational Model
Data Modeling-Part 3
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Objectives
List five properties of relations
State two properties of candidate keys
Define first, second, and third normal form
Describe problems from merging relations
Transform E-R and EER diagrams to relations
Create tables with entity and relational integrity constraints
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relational model?
The relational data model represents data in the form of tables.
The relational model has a solid theoretical foundation. However, we need only a few simple concepts to describe the relational model. It can be easily understood and used even by those unfamiliar with the underlying theory.
Components?
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Data structure
Tables (relations), rows, columns
Data manipulation
Powerful SQL operations for retrieving and modifying data
Data integrity
Mechanisms for implementing business rules that maintain integrity of manipulated data
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Components of relational model
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Relation
A relation is a named, two-dimensional table of data.
A table consists of rows (records) and columns (attribute or field).
Requirements for a table to qualify as a relation:
It must have a unique name.
Every attribute value must be atomic (not multivalued, not composite).
Every row must be unique (can’t have two rows with exactly the same values for all their fields).
Attributes (columns) in tables must have unique names.
The order of the columns must be irrelevant.
The order of the rows must be irrelevant.
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Correspondence with E-R Model
Relations (tables) correspond with entity types and with many-to-many relationship types.
Rows correspond with entity instances and with many-to-many relationship instances.
Columns correspond with attributes.
NOTE: The word relation (in relational database) is NOT the same as the word relationship (in E-R model).
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Key Fields
Keys are special fields that serve two main purposes:
Primary keys are unique identifiers of the relation. Examples include employee numbers, social security numbers, etc. This guarantees that all rows are unique.
Foreign keys are identifiers that enable a dependent relation (on the many side of a relationship) to refer to its parent relation (on the one side of the relationship).
Keys can be simple (a single field) or composite (more than one field).
Keys usually are used as indexes to speed up the response to user queries (more on this in Chapter 5).
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Primary Key
Foreign Key (implements 1:N relationship between customer and order)
Combined, these are a composite primary key (uniquely identifies the order line)…individually they are foreign keys (implement M:N relationship between order and product)
Figure 4-3 Schema for four relations (Pine Valley Furniture Company)
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Integrity Constraints
Domain Constraints
Allowable values for an attribute (See Table 4-1 in next page)
Entity Integrity
No primary key attribute may be null. All primary key fields MUST have data.
Null: A value that may be assigned to an attribute when no other value applies or when the applicable value is unknown.
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Domain definitions enforce domain integrity constraints.
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Integrity Constraints
Referential Integrity–rule states that any foreign key value (on the relation of the many side) MUST match a primary key value in the relation of the one side. (Or the foreign key can be null)
For example: Delete Rules
Restrict–don’t allow delete of “parent” side if related rows exist in “dependent” side
Cascade–automatically delete “dependent” side rows that correspond with the “parent” side row to be deleted
Set-to-Null–set the foreign key in the dependent side to null if deleting from the parent side not allowed for weak entities
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Figure 4-5
Referential integrity constraints (Pine Valley Furniture)
Referential integrity constraints are drawn via arrows from dependent to parent table
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Figure 4-6 SQL table definitions
Referential integrity constraints are implemented with foreign key to primary key references.
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Transforming EER Diagrams into Relations
Mapping Regular Entities to Relations
Simple attributes: E-R attributes map directly onto the relation
Composite attributes: Use only their simple, component attributes
Multivalued Attribute: Becomes a separate relation with a foreign key taken from the superior entity
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(a) CUSTOMER entity type with simple attributes
Figure 4-8 Mapping a regular entity
(b) CUSTOMER relation
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(a) CUSTOMER entity type with composite attribute
Figure 4-9 Mapping a composite attribute
(b) CUSTOMER relation with address detail
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Figure 4-10 Mapping an entity with a multivalued attribute
One–to–many relationship between original entity and new relation
(a)
Multivalued attribute becomes a separate relation with foreign key
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Transforming EER Diagrams into Relations (cont.)
Mapping Weak Entities
Becomes a separate relation with a foreign key taken from the superior entity
Primary key composed of:
Partial identifier of weak entity
Primary key of identifying relation (strong entity)
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Figure 4-11 Example of mapping a weak entity
a) Weak entity DEPENDENT
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NOTE: the domain constraint for the foreign key should NOT allow null value if DEPENDENT is a weak entity
Foreign key
Composite primary key
Figure 4-11 Example of mapping a weak entity (cont.)
b) Relations resulting from weak entity
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Transforming EER Diagrams into Relations (cont.)
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 with the primary keys of the two entities as its primary key
One-to-One–Primary key on mandatory side becomes a foreign key on optional side
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Figure 4-12 Example of mapping a 1:M relationship
a) Relationship between customers and orders
Note the mandatory one
b) Mapping the relationship
Again, no null value in the foreign key…this is because of the mandatory minimum cardinality.
Foreign key
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Figure 4-13 Example of mapping an M:N relationship
a) Completes relationship (M:N)
The Completes relationship will need to become a separate relation.
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new intersection relation
Foreign key
Foreign key
Composite primary key
Figure 4-13 Example of mapping an M:N relationship (cont.)
b) Three resulting relations
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Figure 4-14 Example of mapping a binary 1:1 relationship
a) In charge relationship (1:1)
Often in 1:1 relationships, one direction is optional
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b) Resulting relations
Figure 4-14 Example of mapping a binary 1:1 relationship (cont.)
Foreign key goes in the relation on the optional side,
matching the primary key on the mandatory side
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Transforming EER Diagrams into Relations (cont.)
Mapping Associative Entities
Identifier Not Assigned
Default primary key for the association relation is composed of the primary keys of the two entities (as in M:N relationship)
Identifier Assigned
It is natural and familiar to end-users
Default identifier may not be unique
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Figure 4-15 Example of mapping an associative entity
a) An associative entity
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Figure 4-15 Example of mapping an associative entity (cont.)
b) Three resulting relations
Composite primary key formed from the two foreign keys
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Figure 4-16 Example of mapping an associative entity with
an identifier
a) SHIPMENT associative entity
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Figure 4-16 Example of mapping an associative entity with
an identifier (cont.)
b) Three resulting relations
Primary key differs from foreign keys
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Transforming EER Diagrams into Relations (cont.)
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
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Figure 4-17 Mapping a unary 1:N relationship
(a) EMPLOYEE entity with unary relationship
(b) EMPLOYEE relation with recursive foreign key
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Figure 4-18 Mapping a unary M:N relationship
(a) Bill-of-materials relationships (M:N)
(b) ITEM and COMPONENT relations
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Transforming EER Diagrams into Relations (cont.)
Mapping Ternary Relationships
One relation for each entity and one for the associative entity
Associative entity has foreign keys to each entity in the relationship
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Figure 4-19 Mapping a ternary relationship
a) PATIENT TREATMENT Ternary relationship with associative entity
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b) Mapping the ternary relationship PATIENT TREATMENT
Remember that the primary key MUST be unique.
Figure 4-19 Mapping a ternary relationship (cont.)
This is why treatment date and time are included in the composite primary key.
But this makes a very cumbersome key…
It would be better to create a surrogate key like Treatment#.
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Transforming EER Diagrams into Relations (cont.)
Mapping Supertype/Subtype Relationships
One relation for supertype and for each subtype
Supertype attributes (including identifier and subtype discriminator) go into supertype relation
Subtype attributes go into each subtype; primary key of supertype relation also becomes primary key of subtype relation
1:1 relationship established between supertype and each subtype, with supertype as primary table
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Figure 4-20 Supertype/subtype relationships
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Figure 4-21
Mapping supertype/subtype relationships to relations
These are implemented as one-to-one relationships.
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Copyright © 2011 Pearson Education, Inc. Publishing as Prentice Hall
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