It has properties that are invariant over time

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Extended Overlap Preservation Source-specific elements associated with overlapping elements from the source are passed to the database schema. standardization Independent entities and relationships in the source data should not be grouped together in the same relationship in the database schema. In particular, source-specific elements should not be grouped with overlapping schema elements if the groups co-locate independent entities or relationships. minimalism If elements of the database schema are deleted, the database schema becomes less than ideal. Schemas are stored in the *Data Dictionary, and are also called metadata. (*Data Dictionary: A special type of database containing metadata managed according to a dictionary system. Expresses the nature of data used to design, monitor, document, protect, and control information systems and data within the database. do.


It is a representation of a specific part of the real world and is created using a specific data model.It refers Cambodia Phone Number Data to the structural characteristics of data and is specified by *instance.(*Instance: Refers to forming a tangible value or entity for an abstractly defined concept) 3 layers of schema Schema's 3 Layers Description Image Hedley Digital 3-tier descriptive image of the schema Schema divides a database into three levels according to the user's perspective: external schema, conceptual schema, and internal schema. This is called the 'three layers of schema' . Some say it has a three-layer structure, which will be discussed in detail below, but to explain it very simply: 1. External schema that describes the logical structure from the user's perspective, 2. Internal schema that describes the logical structure in terms of the physical storage device. , 3. Refers to a conceptual schema that describes the logical structure from the perspective of an organization or institution. The three layers of schema can be considered the most important core concept in database schema.




External Schema External Schema Description Image Hedley Digital External schema description image It is said that the three layers of schema are divided according to the user's perspective. The first is the external schema, which defines a part of the organization in the form of a database from the programmer's or user's perspective (perspective). Represents data accessed by an end user or application and provides customized information to the end user. External schema is also called 'subschema' or 'user view', which are all the same thing. It is an ‘outward concept’ that allows you to access the database from that perspective. Since the external schema is based on the user's perspective, it is output differently depending on the user, and there are multiple schemas. For example, a user wants to get information about something from each perspective. When trying to get general information about <driver's license test>, some users will search for driver's license test, others will search for driver's license, and others will search for driver's license written test.