問74
頻出度★☆☆
データベースの論理的構造を規定した論理データモデルのうち, 関係データモデルの説明として適切なものはどれか。
平成26年度 秋期 ITパスポート試験 問74
テクノロジ系データベース › データベース方式
This question's explanation and your answers so far.
解説
問 74
Incorrectア
Representing data (attributes) together with the procedures that process them (methods) bundled into a single object is a description of the object-oriented data model (object-oriented database). A table in the relational data model only holds collections of values; it does not adopt the idea of holding processing procedures unified with the data, so this is not an appropriate description of the relational data model.
Incorrectイ
Representing the relationships among data as a state interconnected like a net is a description of the network model. The network model allows one child to have more than one parent and links records to each other with pointers in a many-to-many fashion. The relational data model relates tables to each other by matching item values, so a description of a mesh-like link structure does not apply to it.
Incorrectウ
Representing the relationships among data with a tree structure is a description of the hierarchical model (hierarchical data model). The hierarchical model represents data as a tree of parent-child relationships in which one child can have only one parent. The relational data model has no concept of a parent-child hierarchy; it represents data as a collection of tables that are all on an equal footing, so this description is incorrect.
- エ
正解 The correct answer is エ “It represents collections of data in a tabular (table) form.” The relational data model is a logical data model that represents collections of data in the form of tables made up of rows and columns. One table represents one kind of subject (for example, employees or products); a column (attribute or field) corresponds to an item name, and a row (record or tuple) corresponds to an individual piece of data. Tables are related to each other by matching values in a shared item (such as the department code in an employee table and the department code in a department table), so there is no need to physically link them in advance with pointers, and adding a table or an item later is unlikely to break the existing structure. This flexibility, together with the fact that relational operations on tables (projection, selection, join) can be written concisely in SQL, is what made the model highly regarded, and it is the basis of the RDBMS (relational database management system) that is widely used today. Note that a logical data model does not define how data is actually recorded (the physical structure); it defines the logical structure of how the relationships among data are understood. Besides the relational model, there are also the hierarchical model, the network model, and others.
ここがポイント
- Memorize the three main categories of logical data models as a set of correspondences: relational model = tabular form (rows and columns), hierarchical model = tree structure (only one parent), network model = mesh-like form (multiple parents allowed). In addition, the object-oriented data model represents things as objects that bundle data and processing together.
- Because the relational data model relates tables to each other through the values of shared items, it stands up well to changes in structure and allows flexible searching with SQL. The system that implements this model is the RDBMS (relational database management system), which is the most widely used type of database today.
用語