3.4.2 Database system - logical structure design: logical structure design process, relational mode conversion
Logical Structure Design Process
Conversion of ER diagram to relational schema:
- Entity to Relational Model Conversion
- Conversion of relational to relational schemas
Generalization of relational schema:
I will talk about this later, there are more contents
Determination of integrity constraints (ensure data correctness)
determination of user attempts (increase data security and independence):
- Determine the view used by the process from the data flow diagram
- Determine which views are used by different users based on user category
application design
Relational Schema Transformation
An entity type must be converted into a relationship mode.
The relationship mode is divided into two types:
there are two ways to convert a one-to-one relationship:
① Independent relationship mode: incorporate the primary key at both ends and the properties of the relationship itself. (Primary key: primary key at either end)
②Merge (at either end): Merge into the primary key at the other end and contact its own attributes. (primary key: remain unchanged)
There are two ways to convert a one-to-many relationship:
①Independent relationship mode: incorporate the primary keys at both ends and the attributes of the relationship itself. (Primary key: multi-terminal primary key)
②Merge (multi-terminal): Merge into the primary key of the other end and contact its own attributes. (primary key: remain unchanged)
There is only one way to convert a many-to-many relationship:
an independent relationship model: incorporate the primary keys at both ends and the attributes of the relationship itself. (Primary key: composite key of primary keys at both ends)
| contact type | Entity (independent relational schema) |
Contact (independent relational schema) |
Contact (Merge Relationship Schema) |
Remark |
|---|---|---|---|---|
| 1 to 1 | √ | √ | √ | merge into either end |
| 1 to many | √ | √ | √ | Incorporate into multi-terminal |
| many to many | √ | √ | × |
For example, the following question:
In the design of the logical structure of the database, the conversion of the ER model into a relational model should follow the relevant principles. For the many-to-many relationship m:n:p between three different entity sets, at least (4) relationship modes can be converted.
As an entity type, ABC must be converted to an independent relational schema, so there are 3 relational schemas here
Looking at the contact mode again, the three are directly many-to-many, so the relationship mode cannot be merged, and only three of them share one relationship in the figure, so the contact mode is converted into one relationship mode. ABC will not be considered separately, because the connection has been abstracted, and ABC is connected to the same connection.
A total of 4 are needed.