ER Modelstands for Entity Relationship Model is a high-level conceptual data model diagram. ER model helps to systematically analyze data requirements to produce a well-designed database. The ER Model represents
ER modeling/ C6160 Database management systems (DBMS) C6160J Object-oriented databasesThe Entity-Relationship (ER) model allows a database designer to develop a high-level conceptual schema without having to consider low-level issues such as efficiency, the underlying database management system ...
Key Constraint: Every relation has a designated attribute (or a concatenation thereof) which acts as the primary key for the relation in the sense that the value of its primary key identifies the tuple uniquely. 3. Entity Integrity Constraint: No primary key value can be the distinguished NULL...
Weak entity is represented bydouble rectangle. The relation between one strong and one weak entity is represented by double diamond. Weak entities are represented with double rectangular box in the ER Diagram and the identifying relationships are represented with double diamond. What is difference betw...
In a DBMS, an attribute refers to a characteristic of an entity about which data is collected and stored. Attributes form an essential component of a...Become a member and unlock all Study Answers Start today. Try it now Create an account Ask a question Our experts can answer y...
Entity Entity : An entity can be a person, place, event, or object that is relevant to a given system. For example, a school system may include students, teachers, major courses, subjects, fees, and other items. Entities are represented in ER diagrams by a rectangle and named using singu...
Entity: entities are instances of Entity Types, which are richly structured records with a key. Entities are grouped in Entity-Sets. 这里提到实体需要一个Key,这里就相当于对应数据库类型中的主键,在上面的ItemInfo中,这个主键就是Id这个属性。
Case Study – Internet Shop I then prepare a shipment that contains the books they ordered. If I don't have enough copies in stock, I order additional copies and delay the shipment until the new copies arrive; I want to ship a customer's entire order together. My catalog includes all th...
A relationship set is a mathematical relation among n 2 entity sets. {(e1, e2, … en) | e1 E1, e2 E2, …, en En} where (e1, e2, …, en) is a relationship Example: (Hayes, A-102) depositor Relationship Sets (Cont.) an attribute can also be propert...
What are the additional constraints in this diagram? Binary vs. Ternary Relationships Previous example illustrated a case when two binary relationships were better than one ternary relationship. An example in the other direction: a ternary relation Contracts relates entity sets Parts, Departments and Su...