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This paper introduces the concept of data model CDM PowerDesigner in the basic concepts.
a conceptual data model data model overview
data characteristics in the real world of abstraction. Data model should meet three requirements:
1) to compare realistic simulation of real-world
2) easily understood by human
3) to facilitate the concept of the computer realization of the data model also
said information model, it entities – Contact (Entity-RelationShip, referred to as ER) theory, and this theory was expanded. It from the user point of view the information modeling, the concept is mainly used for database-level design.
real world people usually abstract the concept of first world, then the concept of the world into the machine world. In other words, is the first objective real world objects in abstract entities (Entity) and contact (Relationship), it does not depend on the specific computer system or a DBMS system, this model is what we call the CDM; CDM then converted to computer supported by a DBMS data model, this model is the physical data model, which PDM.
CDM is a group of well-defined set of model elements, elements of these models accurately describe the system static characteristics, dynamic characteristics and integrity constraints, etc., including data structures, data manipulation and integrity constraints c section.
1) data structure represented as entities and attributes;
2) expressed as entity data manipulation of records to insert, delete, modify, query and other operations;
3) expressed as integrity constraints their own data integrity constraints (such as data types, inspection, rules, etc.) between the reference and data integrity constraints (such as contact, inheritance links);
Second, the entities, attributes and identifiers defined
entities (Entity), also known as instances, corresponding to the real world can be distinguished from other objects of the “event” or “things.” For example, students in each school, every hospital in the surgery.
Each entity has used to describe the physical characteristics of a group of properties, called attributes, an entity described by a number of attributes. If a student entity by student number, name, sex, date of birth, where the Department of gender, school year and other attributes.
entity sets (Entity Set) is the specific nature of the same type and the same set of entities. For example, the set of all school students be defined as “students” entity set, “Student” entities Each entity set has a student number, name, sex, date of birth, where the Department of gender, school year and other properties.
Entity Type (Entity Type) is a centralized entity for each entity, the common nature of the collection, such as “patients” entity type are: {out-patient number, name, sex, age, identity card number.}. Entity is an instance of entity type, the meaning clear in the case of entities, entity types are usually used interchangeably.
entity type that uniquely identifies each entity that contains one or a group of properties known as the entity type identifier (Identifier), such as “Student ID” is the identifier of the entity type of students, ” Name “,” Date of Birth “,” the letter addresses the “common form” citizens “entity type identifier.
some type of entity can have several sets of attributes as the identifier, the selected one group of attributes as the primary identifier of an entity type, the other as the secondary identifier.
three entities, attributes and identifiers of the expression
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