Transcript Weak Entity Sets

Databases Revision
1.1
Database and DBMS
 Database: Set of contiguous data that is connected together using
relations, and stored without redundancy.
 DBMS
 DBMS Users:
 DBA: define S.W. requirements, Maintainability, Consistency,
Security, Authorities, Monitoring.
 DB Designer: To define tables and relationships without
redundancy
 DB Programmer: To create DB, forms, reports,…etc.
 Users.
1.2
DB Generations
 First Generation: Tree Data Model and Network Model.
 Second Generation: Relational Data Model.
 Based on relational algebra
 Some data types could not be represented (Multimedia)
 Third Generation: Enhanced RDBS.
 Fourth Generation: Object Oriented Model (OMT).
1.3
DB Keys
 Super Key: Set of columns each of which has a unique
data.
 Candidate Key: Any one of the super key elements.
 Primary Key: The selected candidate key for some table.
 Composite Key: The primary key is composed of more than
one field.
 Foreign Key: Refers to the primary key of other table.
1.4
Entity-Relationship Model
 DB Model: Tool to describe data, relationships,
semantics, and constraints of a DB.
 We use Entity Relational Model with relational
databases.
 ER Model lies in the conceptual layer.
1.5
Entity-Relationship Model
 Example:
Degree of depositor relationship = 2
Degree of Entity account = 2
Degree of Entity customer = 4
1.6
Entity-Relationship Model
 The Schema: The logical structure of the database. (Tables,
relations,…etc)
 The Instance: Content of DB at some time.
 Domain: The set of permitted values for each attribute.
 Cardinality: The number of entities in the entity set that
another entity can be associated with using relationship.
 One to One
 One to Many
 Many to Many
1.7
Entity-Relationship Model
 Rectangles represent entity sets.
 Diamonds represent relationship sets.
 Lines link attributes to entity sets and entity sets to relationship sets.
 Ellipses represent attributes
 Double ellipses represent multivalued attributes.
 Dashed ellipses denote derived attributes.
 Underline indicates primary key attributes (will study later)
1.8
Example
1.9
Roles
 Entity sets of a relationship need not be distinct
 The labels “manager” and “worker” are called roles; they specify how
employee entities interact via the works-for relationship set.
 Roles are indicated in E-R diagrams by labeling the lines that connect
diamonds to rectangles.
 Role labels are optional, and are used to clarify semantics of the
relationship
1.10
Cardinality Constraints
 We express cardinality constraints by drawing either a directed
line (), signifying “one,” or an undirected line (—), signifying
“many,” between the relationship set and the entity set.
 E.g.: One-to-one relationship:
 A customer is associated with at most one loan via the relationship
borrower
 A loan is associated with at most one customer via borrower
1.11
One-To-Many Relationship
 In the one-to-many relationship a loan is associated with at most
one customer via borrower, a customer is associated with
several (including 0) loans via borrower
1.12
Many-To-One Relationships
 In a many-to-one relationship a loan is associated with several
(including 0) customers via borrower, a customer is associated
with at most one loan via borrower
1.13
Many-To-Many Relationship
 A customer is associated with several (possibly 0) loans
via borrower
 A loan is associated with several (possibly 0) customers
via borrower
1.14
Participation of an Entity Set in a
Relationship Set
 Total participation (indicated by double line): every entity in the entity
set participates in at least one relationship in the relationship set
 E.g. participation of loan in borrower is total
 every loan must have a customer associated to it via borrower
 Partial participation: some entities may not participate in any
relationship in the relationship set
 E.g. participation of customer in borrower is partial
1.15
Weak Entity Sets
 An entity set that does not have a primary key is referred to as a
weak entity set.
 The existence of a weak entity set depends on the existence of a
identifying entity set
 it must relate to the identifying entity set via a total, one-to-many
relationship set from the identifying to the weak entity set
 Identifying relationship depicted using a double diamond
 The discriminator (or partial key) of a weak entity set is the set of
attributes that distinguishes among all the entities of a weak
entity set.
 The primary key of a weak entity set is formed by the primary key
of the strong entity set on which the weak entity set is existence
dependent, plus the weak entity set’s discriminator.
1.16
Weak Entity Sets (Cont.)
 We depict a weak entity set by double rectangles.
 We underline the discriminator of a weak entity set with a
dashed line.
 payment-number – discriminator of the payment entity set
 Primary key for payment – (loan-number, payment-number)
1.17
Database Integrity
 DB Integrity: Accuracy and Correctness of Data
 DB Integrity Types:
 Domain Integrity Constraint:
– CREATE DOMAIN stno AS CHAR(5)
SUBSTR(stno,1,1) = ‘s’ AND IS- NUMERIC(SUBSTR(stno,2,4))
– CREATE DOMAIN location AS CHAR(15)
location in( ‘Irbid’ , ‘Amman’ , ‘Aqaba’);
– CREATE DOMAIN stno AS INTEGER(3)
stno > 0
 Domain Forms Integrity:
– Telephone Number Field.
 Domain Range:
– Grade field must be between 0 and 100
 Entity Integrity: Any primary key in some entity must not be null
 Referential Integrity:
– Do not add to detailed table till you add to the master one first.
– Do not delete from master table until you delete from detailed one.
1.18