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 real-world entities and the relationships between them. Creating an ER Model inDBMSis ...
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...
If a DBMS enforces an UPDATE RESTRICT option on the referential integrity constraint between CLIENT and AGENT in the HAPPY INSURANCE database, what will be the outcome after a user tries to change the 1. To create a relationship between two tables, you need at least a data type. q...
The integrity rules are used for algorithmic design of integrity-preserving update procedures, to be utilised in database application programs, instead of the primitive update operations supported by the DBMS itself. These procedures may be generated directly from the specification, opening the way to...
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...
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。
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...
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...
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...
Specify the name of the sequence table in the generator-name element. Specify the size of the key cache— how many keys the container will fetch in a single DBMS call—in the key-cache-size element. BEA recommends a key-cache-size greater than one. This setting reduces the number of ...