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In development of system specification and modeling is to analyze the structure of information. In order to create such model for our data structure,there is used the most often conceptual diagrams (conceptual).
In this lesson compounds I will present diagram of ERD entities (entity relationship diagram) which is equivalent to a graphical model of relationship entities ERM (entity relationship model).
ERD helps graphic modelling of data structures and the relationships between them. In this way, you can easily design a structure with ease with all attributes necessary for optimal functioning. If you already have a finished diagram ERD using the CASE system (which also helps in creating diagrams) ,we can often generate a ready-made database.
Diagramming conventionsBelow is presented list of alternative diagramming convention:
- Bachman's notation
- Martin's notation
- The (min,max) notation of Jean-Raymond Abrial
- UML notation
- EXPRESS notation
- Chen notation
- Crow's Foot
Basic conceptsTo create diagram it should be understood following concepts:
entity - An entity is something that has a distinct, separate existence, though it need not be a material existence. In particular, abstractions and legal fictions are usually regarded as entities. In general, there is also no presumption that an entity is animate. :
- attribute - element ofinformation for classification, identification, eligibility,
determining the amount or expression of the state of entities. It may be a number, text, logical value or image.
Examples of attributes for the entity - person
- relation - This is a link between two or more entities. The links in the diagram can be described by size, optionality and the relationship between the entities.
- grade of relation :
- optionality of the relation :
optional relationmandatory relation