Entity-Relationship Data Model CS 157A Professor Sin-Min Lee Student: Yen-Chu Pan

Elements of E-R Model In the E/R model, the structure of data is represented graphically, as an “entity-relationship diagram,” using three Principal element types: Entity Sets. An entity is an abstract object of some sort, and a collection of similar entities forms an entity set. Attributes The properties of the entities in the set. Relationship The connections among two or more entity Sets

Example of Elements of E-R Model Entity Sets Departments Professors Students Administrators Attributes Name of Departments, Phone No., Address... Name, SSN, Address of Professors... Relationship Students and Professors are under a certain department Admin manage the campus/ departments

Example of the 3 elements in E/R Diagram

Classification of Constraints Keys Single-value constraints Multi-valued constraints Mapping Cardinalities and Participation Constraints

Key in the E/R Model Superkey is a set of one or more attributes that, taken collectively, for us to identify uniquely an item in the entity set. For example, customer-id is a superkey. Candidate key is a minimal superkey. For example, customer-name and customer-street is sufficient to distinguish among members of the customer entity set. Then {customer-name, customer-street } is a candidate key. Primary key denotes a candidate key that is chosen by the database designer as the principal means of identifying items within an entity set. the primary key should be chosen such that its attributes are never, or very rarely, changed. For example, Social-security numbers are guaranteed to never changed.

Single/Multi-valued attributes Single-valued attributes are attributes that only have a single value for a particular entity. Multi-valued attributes refers to items that are not singled-value and Null valued. For example, consider an employee entity set with the attribute phone-number. An employee may have zero, one, or several phone numbers; different employee may have different numbers of phones.

Mapping Cardinalities or Cardinality ratios Express the number of items to which another item can be associated via a relationship set Are most useful in describing binary relationship sets. For a binary relationship set R between entity sets A and B, the mapping cardinality must be one of the following: One to One One to Many Many to One Many to Many

Participation Constraints The participation of an entity set E in a relationship set R is said to be total, if every item in E participates in at least one relationship in R. If only some items in E participate in relationship R, the participation of entity set E in relationship R is said to be partial.

Weak Entity Sets There is an occasional condition in which an entity set’s key is composed of attributes some or all of which belong to another entity set. Such an entity set is called a weak entity set.

Discriminator The discriminator of a weak entity set is a set of attributes that allows this distinction to be made. For example, the discriminator of a weak entity set payment is the attribute payment-number, since, for each loan a payment number uniquely identifies one single payment for that loan. The discriminator of a weak entity set is also called the partial key of the entity set.

Requirements for Weak Entity Sets We cannot obtain key attributes for a weak entity set indiscriminately. Rather, if E is a weak entity set then its key consists of: Zero or more of its own attributes, and Key attributes from entity sets that are reached by certain many-one relationship from E to other entity sets. These many-one relationship are called supporting relationships for E.

Discriminator (cont.) Note: although each payment entity is distinct, payments for different loans may share the same payment-number. Thus, payment entity set does not have a primary key; it is a weak entity set. The primary key of a weak entity set is formed by the primary key of the identifying entity set, plus the weak entity set’s discriminator.

Identifying a Weak Entity Type A Weak entity type doesn’t have a primary key. If X is a weak entity type and Y is the entity type on which X is dependent. We form a primary key for X by combining the primary key of Y which one or more attributes, called discriminator or partial key, from X. In an E/R Diagram, a partial key is usually dash-underlined. e.g., primary key for DEPENDENT: {Employee No., DName}.

* * Doted-line = double-line

References Peter Chen’s website: http://bit.csc.lsu.edu/~chen/chen.html Database Systems: A First Course, J.D. Ullman & J. Widom http://www-db.stanford.edu/~ullman/fcdb.html http://www.cs.sjsu.edu/faculty/lee/cs157/25Sp157AL4.ppt http://www.cs.sjsu.edu/faculty/lee/cs157/25Sp157AL5Enhanced%20ER-diagram.ppt