Schema (database)
The schema (schema) is the structure of the database and is described in abstract language employing precise grammatical constructions supported in database management system (DBMS). By a relational data base, the schema is a related definition of an attribute (field), an attribute and the relations in relations (table) and relations.
The schema is stored away generally by data dictionary. The schema is defined in data definition language (DDL) of the database language with the text, but often calls the thing which expressed database structure graphically a schema [1].
I customize an attribute (field), and I am made by a use. For example, an attribute to A - E is defined by one substance and defines A, C, the attribute of E for schema 1, and I define B, D, E, and schema 2 can realize what I apply on the basis of two kinds of data.
Table of contents
Three levels of schema
Three levels of schemata (Three schema approach) include some types. For example, there are a thing to divide into three of concept - logic - physics and a thing to divide into three in outside - concept -
Concept - logic - physics method
- Definition of the relations between concept schema - concept and concepts
- Logic schema - substance and the attribute, definition of relations between the substance
- Concrete implementation of physical schema - logic schema
Concept schema
I abstract information necessary for a database by data model, and the concept schema (British: Conceptual schema) is the description that defined the relations between concepts as the concept that I abstracted. I explain the semantics of the target organization and express a series of logical expressions about the property. Particularly, I explain the information associated with it, the connection between those characteristic (attribute) things that it is the intentionality of the organization (substance class).
Because I express the semantics of the organization and am not a database design, the abstract level varies. In four levels of schema architecture of ANSI, I call the concept schema that reflected each user's original point of view "an outside schema". On the contrary, it is said that the thing which gathered them is "a concept schema". The data model is fixed at the point of view of each user and tends to be lacking in flexibility with that alone. If the personal view of the world changes, the model is forced to a change, too. The concept schema is abstract; essential.
For a concept schema, I am equivalent to the succession to say in object oriented
もある. I can classify the sets of the instance of a certain substance class in the suitable lower substance class. Therefore, instance of a certain lower substance class is instance of the higher substance class. Instance of the higher substance class is instance of some kind of lower substance classes. .
The pattern / derivation type relations of "exclusive relations" may be. The instance of a certain pattern may have to be the instance of the derivation type of one one methodologically. Similarly, I may be exhaustive relations. When it is cyclopedic, instance of a certain pattern must be instance of some kind of derivation type by all means.
Logic schema
The logic schema (British: Logical schema) is data model described assuming a specific data management technology about a target domain. But a model concerned describes, for example, relations (table) and a group (tuple, line), object model using a class or an XML tag without depending on the specific database management product. It is in contrast to describing the semantics of the organization without a concept schema considering an implementation technology really.
After having made a logic schema when I make a database, I make a physical schema from there.
Physical schema
The physical schema (British: Physical schema) is data model described at the point of view of the specific data management technology. I call it "an internal schema" in four levels of schema architecture of ANSI. The logic schema defines the logical expression of data, but the part that how data are stored away concretely by a storage (auxiliary memory) is not considered at this stage. The physical schema defines the data storage in the individual database management product concretely.
Other method
In ANSI/X3/SPARC (SPARC is Standards Planning And Requirements Committee: standardization plan committee), I use three levels of schemata of a concept schema, an outside schema, the internal schema. I assume it three of a vice-schema, a schema, the memory schema in CODASYL which developed the COBOL that is often used with a general-purpose computer.
| Use in ANSI/X3/SPARC | The left program | Definition of supporting CODASYL |
|---|---|---|
| Outside schema | I took necessary information out of concept data (if it is RDB, I am equivalent to View) | sub-schema (vice-schema) |
| Concept schema | Logic data (if it is RDB, I am equivalent to a table) on DB | schema (schema) |
| Internal schema | Method to manage the data in DB | Storage schema (memory schema) |
Allied item
Outside link
- Different point of view in the community of the Data Modeling 101 アジャイルソフトウェア development
This article is taken from the Japanese Wikipedia Schema (database)
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