Contact two types 1. The contact between the inside of the entity, such as the connection between the properties of the entity 2. The connection between the entity, divided into three categories a) 1 pair 1 Contact B) 1 pair more Contact C) Multi-to-many contact ER diagrams design process 1. Determine the entity set 2. Determine the contact and contact type 3. Model 4 represented by the ER diagram is designed. Determine attribute 5. Optimize the classification of the structural model 1. Hierarchical model features: there is only one node unparalleled, this node is called the other nodes of root nodes, and only one double pro-first layer and the next layer of recording are 1 to more shortcomings: can't represent The complex contact between two or more real types and many-to-many links between the physical types 2. Net model features: There is more than one junction that there is no double-pro-point point to more than one doubrowless disadvantage: writing applications compare complexity need to be familiar with the logical structure of the database 3. Relationship model relationship is the main term 1 in two-dimensional table relationship model. Relationship: An ecosystem corresponds to a two-dimensional table 2 we usually talk about. Tuan Group: One line in the table is called a tuple 3. Properties: One column in the table is called attributes, and the names of each column are attribute name 4. The main code: a attribute group in the table, and their value is uniquely identified a tuple 5. Domain: Value range of attributes 6. Component: One attribute value in the tuple 7. Relationship mode: Description of the relationship, use the relationship name (attribute name 1, attribute 1 2, ..., attribute name 3): 1. Concept single 2. The relationship must be a standardized relationship 3. The user's retrieval operation of the data is just from the original table to get a new surface to object model object: the real world of the real world, the record is complex, each object has a unique identifier, and the status and behavior package stand up. The status is a collection of attributes, the behavior is a method of operating on an object state. Class: combine all objects that are the same as the method set, can form a class
