[SQLD] 3. Understanding data modeling

data modeling

Business analysis technique from a data perspective to build an information system.

Things to keep in mind about data modeling include duplication, inflexibility, and inconsistency. Data or attributes should not be stored redundantly to prevent the database from making the mistake of storing the same information in multiple places. Depending on how the data model is designed, even minor changes can cause the data model to change frequently, making maintenance difficult. Therefore, it must be designed taking into account the fact that it can flexibly change according to future changes. In addition, relationships between data must be clearly defined to avoid updating related data.

schema

Describes the overall specifications of the structure and constraints of the database. The DBMS creates a database schema according to the given settings, and when a database user stores, searches, deletes, or changes data, the DBMS executes commands by referring to the database schema it created.

External schema: Defining part of the organization in the form of a database from the perspective of a programmer or user, the user's view

Conceptual schema: Integrated expression of the entire organization’s perspective incorporating all user perspectives

Internal schema: Describing the physical storage format of the entire database

entity

In a database, an entity means an object, something to be stored. It is a meaningful unit of information in the world of human concepts or information. In ERD, entities are placed in the upper left corner, and the most important entities are placed in the center below, so that the lines can be placed effectively without getting twisted.

The entity must be information that is to be managed in the relevant task. And it must be identifiable by a unique identifier. It must be a set of two or more instances that exist permanently. It must be used by a business process and must have properties. There must be at least one relationship with another entity.

relationships between entities

Relationship degree: This indicates the cardinality of the relationship and is expressed as 1:1, 1:M, M:N.

Relationship Options: Required Relationship, Optional Relationship

You must check whether there are verbs in the job description and form that enable relationships.

attribute

The smallest unit of data that can no longer be semantically separated from the instances required for business.

One attribute has one attribute value. It is best to ensure that the name of the attribute is unique across the entire data model.

One entity (table) is a set of two or more instances (rows).

One entity has two or more properties (columns).

The entity is a 2*2 table

domain

There is a range of values that each attribute can have, which is called the domain of that attribute, and specifies the data type, size, and constraints for the attribute within the entity.

Example) The book name is defined as a string of less than 100 characters.

Identifiers and Deidentifiers

The identifier expresses a strong connection and is included in the composition of the stock identifier. It is expressed as a solid line and is necessarily dependent on the parent entity. It is necessary to transfer inherited stock-specific properties to other TT. In other words, in order to receive the parent entity properties from the child entity and continue to pass them down to the grandchild entity, the identifier relationship is considered.

On the other hand, in non-identifier relationships, consider non-identifier relationships when you want to have an independent primary key structure in the child table.

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